The content of many stories can be attributed to American ambivalence toward major changes in society associated with industrialization: manufactured foods, large impersonal organizations, urbanization, new technology, etc.
In particular, corporations are portrayed in legends as:
The largest and highest prestige companies in a product category are the most frequent targets. Even though the worms in hamburgers story originated with Wendy's, the target soon switched to McDonalds. Burger King is almost never mentioned. The targets can be either large or prestigious. For example, the Cadillac stories reflect a high prestige product, not a major seller. A modern example is Microsoft, which is the target of hundreds of rumors.
The degree to which the most visible companies are targeted is way out of proportion to their visibility. One reason why this may occur is the process by which stories are recalled and retold. People do not actually recall most facts, they reconstruct them. So in telling the fried rat story, they know that the story calls for it happening at a fried chicken place, and they guess that it is KFC, because it is the most salient in their minds.
Another reason may be that the underlying function of the story is to expose the transgressions of businesses. In that case, it may make sense to use the market leader because the leader symbolizes the entire category. As we know, it is common practice to refer to products by the name of the dominant brand, such as Xerox machines, Kleenex, and Coke.
So why do people want to expose the transgressions of businesses? Perhaps it reflects an underlying understanding that the business exists only secondarily to serve the customers. It's primary goal is to make money, and it answerable only to it's owners. Therefore, they are capable of doing things that are not in the customer's interests, and the customer has little control over the organization's actions. In a capitalist society, only ownership confers real control.
As national economies become increasingly intertwined, individual Americans are increasingly affected by events in foreign countries. For a nation that was previously rather isolationist, this may be an uncomfortable situation.
The snake in the lining of the coat story reflects the dangers of importing products from exotic foreign countries, especially Asia and South America.
These kinds of rumors are often transmitted deliberately by people with an axe to grind. For example, the rumor that Corona beer was made with urine was actively spread by the sales people from a competing company. They settled out of court and declared publicly that the rumors were false. In wartime, governments actively spread tales about the other side, particularly stories that the other people eat prisoners of war.
Fear and dislike of foreigners is very old and runs very deep in most societies. A lot of stories concern the foreigners themselves: immigrant groups.
Businesses are seen as hard headed, bottom-line bastions of rationality. Yet a common tale is that the heads of these organizations are members of Satanic cults or terrorist fringe groups like the KKK, or that the organization as a whole is owned by these groups.
That these stories are plausible means that people do not find it farfetched that (a) organizations can be deliberately evil, and/or (b) that these cults exert a lot of power over the economy, or (c) the connections between the cults and the organizations can be kept secret.
Conspiracy stories have long history. In the past, it was a common belief that most important companies were secretly controlled by Jews. Americans also commonly believed that the Catholic church was controlling significant portions of the economy.
Proctor & Gamble is associated with Satanism and the Moonies. In 1985, P & G mentioned in a press release that they had received 100,000 calls and letters about that. One reason is the moon & stars logo resembles satanic symbols. In addition, the unnamed pres. of P&G is said to have appeared on Donahue (or similar show) and admitted his involvement in the Church of Satan. This apparently resulting from the collision of the logo issue with an appearance by Ray Kroc of McD's on Donahue mixed with a rumor that McD's was so successful because it had made a pact with the devil.
P & G actively fought the rumors, getting religious leaders to denounce the rumors and prosecuting rumor mongers. Typically, the only way to fight rumors is to spread counter-rumors about the kind of people who are spreading the original rumor. For example, when rumors spread that a secret society of black men were systematically raping white women, causing many black people to be assaulted on the street, the rumor was stopped by spreading the rumor that the first rumor was being spread by ku klux klan members, so that people spreading that rumor became suspected KKK members.
These rumors may reflect a belief that the machinelike corporations can be used by cult groups for their own purposes. Corporations are like weapons which can be used for for good or evil.
Plausibility. Most tales have enough plausibility that they cannot be outright dismissed as false. Furthermore, they are based on or validate fundamental beliefs, like technology is dangerous, or foreigners cannot be trusted, or promiscuity is bad.
Irony/Revelation. Many tales have a sense of irony about them. Often this is expressed as 'things are the opposite of what they seem' or 'what goes around comes around'. For example:
What goes around ..
Status. The "people are not what they seem" usually revolve around high status individuals/organizations.
The stupidity of others. The tales about technological innovations can also be read as saying "see how stupid people are" (e.g. drying off cat in the microwave).