I can't completely blame people for doing this since Touhou is unique in the fact that character development isn't concentrated in any one place. You have to read EVERYTHING to get a good feel for the characters, and that is a rather tall order considering that some of it isn't even translated yet. But most people just push this way past the limit of common sense. The "Reimu is a bitch" thing is by far the worst offender though, considering she's actually probably one of the nicer (if occasionally temperamental, but hey who isn't?) people we've been shown. Marisa is also a lot more even headed than fans give her credit for, especially in the print works.
Hell, there are so many things in the endings that people just plain don't know because they never beat the games. I remember getting the Reimu and Marisa endings in EoSD and being like "Wait, wtf? Sakuya just lets Marisa in and tries to hide her from Remilia?! Why the hell do they make Sakuya out to be murderous bitch to Marisa then? God dammit, this is WAY more interesting than that bullshit they told me". After that I never listened to a god damned thing fans have ever said and decided to form my own opinions.
I sometimes think that we should put together a good introduction to the series and descriptive character profiles based ENTIRELY on canon (instead of conjecture), just to help mitigate the deluge of people that are relatively new and flood the boards with misinformation and stupid questions. I would say we already have the wiki for that, but I think that might be a bit too much for some people to wrap their heads around. Maybe a "Top 100 things fans think about Touhou that are complete bullshit", along with proof.
Damn, I think I rambled on a bit there. Sorry 'bout that.