One factor that leads to things being interpreted as political allegories is that writers read the same newspapers as the rest of us, and have opinions just like everyone else; just because a writer doesn't run for office (like Norman Mailer and Upton Sinclair did) doesn't mean he or she doesn't have political beliefs, and those beliefs often subtly (or not subtly) show up in their works. Another is that many politicians are buffoons, and many fictional politicians are portrayed as buffoons, and there's inevitably overlap in the buffoonery — particularly if the author has a specific politician's antics at the back of his or her mind.
Some countries also have a tradition that intellectuals should largely discuss political issues of important historical events like World War 2 or contemporary events in fiction. Because of this, in these countries, authors of certain genres may also get the reputation of integrating political messages into their works no matter if they actually did so.
Certain political theories and ideologies, such as various forms of Marxism, also adopt a perspective that everything is to some degree political, as the political informs and governs every aspect of our lives in some way, shape, or form. According to this approach, nothing can be truly apolitical, as to be apolitical is to tacitly condone the status quo; even if you disagree with the status quo, by not speaking up against it, you are essentially condoning it, since nothing's going to change.
The other thing is that popular mass media entertainment does not exist in a vacuum. It is inspired by fashions, currents, fads, tastes, and all kinds of novelties. Much of this is ephemera, but some of these ephemera can and do have Unfortunate Implications. Advertising in the 50s and 60s, for example, as well as popular entertainment of that time is riddled with sexism, chauvinism, and racism. At the time, this was not much commented on, and did not receive a great deal of scrutiny. While the question of the influence it had in enforcing stereotypes or if a critical work of art can make a difference is a bigger can of worms than this page can handle, such media can't really claim to be innocent by any means. It was for this reason that a lot of contemporary media, hastily in some cases, is judged politically for stereotypes, assumptions, and Unfortunate Implications.
