So some girls say they want to date nice guys. But they also tell (alleged) nice guys that they just want to be friends. "Nice guys" get frustrated at the inconsistency.
Some "nice guys" then get mad and start bashing the women who want nice-guys-except-you. And some women respond by pointing out that he couldn't have cared all that much about you as a person if he rejects your friendship when you don't want to be romantic. (See here. Warning: crude language.) Of course, if you treat her like crap just because she doesn't want to date you, you aren't actually a nice guy to begin with, so the whole conversation is moot.
But not every nice guy who complains is a secret asshole. I think it depends on how you complain--what you do and say.
And, really, this goes for both genders. I'd think it'd be obvious that you can both care deeply about someone and have romantic feelings for that person. If the romantic feelings aren't reciprocated it can get be quite painful, and I don't think complaining about your pain is an inherently jerk thing to do.
On the internet it seems like all the "nice guys" who complain aren't at all nice anyway, but in reality I've known plenty of guys I considered genuinely nice who struggled in the dating world for other reasons. I can see how that would be really frustrating.