Well,my thought is that the people who do the spamming, don't now it's not allowed so they stick it in whatever section they want.  Having a "Crappity Crap"  forum will not attract many (customers?).  Sure, the mods could move everything,but they're working hard enough right now.
Plus, useless threads don't get many posts, so I don't see much action in that section.  And we don't want anything to soil the good name of this forum.  "Yeah, I was over at PGC, and they have a TON of spam *cough over at the spam section cough*"  
"Pardon?"
"Nothing..."
Polls, games, selling of systems, etc are pretty much useless, and flambait's just make people mad/sad.  There is no need for any of these useless threads, so there is no need to have a place to put them.  The mods do a great job here trying to get us into a good conversation, so we have to think about what we're going to write (unlike I did here 

)  and don't write, "XBACKZ SUXZ" everywhere.  I figure, once we enter that section, our brains would start to drip out of our ears and onto the keyboard, which would short circuit and electrocute us.  So basically, a section like that would cause more deaths due to forum use in the world, and the numbers are quite high now.