Because sweaty ass, piss, and [expletive] are so much cleaner? Even if a toilet seat appears clean/dry, it means absolutely nothing. There is no "my" seat about it. One takes a risk every time they use a public toilet. What's the risk (and this is coming from someone super unwilling to take dumps on public toilets - while realizing how mostly irrational it is)? Herpes? Is that like getting it from a tractor? Nice. Though I would hope everyone is unwilling to dump ON a toilet, public or otherwise.