I think a big part of why Pearl Jam, Nirvana, and obviously Soundgarden (Among others) trash STP in press was because of their status. Let's go back to when they came out. Grunge was storming the nation and all those Seattle bands were suddenly getting huge record sales. Then out of nowhere a California band is signed and given a huge deal to record an album and make it sound "grungy". To some it was a turf thing.
That's not knocking STP's music and I don't want this to turn into a "they worked hard to get where they were! Mighty Joe Young! MIGHTY JOE YOUNG!!!!!!!!!!!!!!!!!!!!!!!". Look at it from some of the bands' point of views and how it all came together: A completely unknown band who had recorded a demo (Remember, this is before the age of internet so aside from some California people who went to local gigs nobody had heard of them) suddenly signs a huge contract after only playing for a little while, under the record label promise they can catch on with the recent "Seattle Grunge" phase. I think some of the bands still take it personally. STP took a lot of the backlash from it, since by the time Candlebox (Who moved to Seattle and played I believe two shows before signing to a major label), Silverchair, etc came around grunge was slowly beginning to drift out of its peak popularity.
I also must commend TemplePilot for saying it best (As usual): A person slanders STP in the tiniest bit and suddenly he's the worst guy ever who knows nothing about real music. Not everybody here is like that, but there definitely are a key few.
~Jeremy~