Guys often ask, "What's the big deal? Why should I have to put the seat down, instead of you having to put it up?" On the surface it seems like a valid question. To answer it once and for all, it has to do with midnight bathroom runs. When we're half asleep, blinded by the tiniest bit of light (so we leave them all off), and about to burst, we aren't lucid enough to think about checking for a seat behind us. (Remember, guys, we don't look at the thing when we use it - we face away.) And when we sit down on cold porcelain and slip backside first into even colder water, well, we just have to yell at the person who left the seat up. If you accidentally pee on a seat that's left down, at the most it's a quick wipe (I say "at the most" because I've had to cleap up after others too many times to count). That just doesn't compare to an icy butt bath, followed by the fear that maybe the last time someone used the toilet they also didn't flush.
To answer your question, however, it's because they either don't truly understand WHY we're so annoyed (and it is hard to understand if you've never experienced it), or else they just don't care. Explain rationally. Don't get "pissy" (sorry, couldn't resist). And if all else fails, write in black permanent marker on the bottom of the seat - "PUT IT DOWN!!!"