I am very active on tumblr and I see people complain about this a lot. Most cisgender, heterosexual people are ignorant of trans* issues and some are downright abusive to us, and there are more cis people than trans people so we generalize. Besides, when someone posts something like, "I was harassed today. #cispeoplesuck" any cis person who is aware and sympathetic of trans* issues won't take offense because they are obviously talking about the bad ones and are just venting their frustrations.
There is, however, the issue of people harassing cis people directly. I know a cis girl on tumblr who's bag was vandalized, when she confronted the culprits they called her a "cishet bitch" and said they didnt need a reason to harass her. That is obviously wrong and definitely needs to stop.