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) in the work. 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 pol's antics at the back of his or her mind.