Some possibilities?
(a) Maybe those fora attract a less tech savvy clientele than most.
(b) Maybe those fora have no idea how many innocent users they've locked out as collateral damage.
(c) Maybe those fora are more actively moderated so you can't spot spammers and serial trolls as easily.
My money is on (c) because, as a former web server administrator, I know for a fact that user=/=IPaddress and IPaddress=/=user. Blocking an IP address just blocks an address that could be assigned to any number of different people at any given time; it doesn't block a specific user.
If blocking an IP happens to cause a user any inconvenience at all, it's trivial and only the most technically illiterate users will be unable to get around the block with ease. Let's just think a moment about any of our usual suspects here at FH.com/forums and guess how long it would take any of them to get around an IP block. (Hint: Not very long at all.)

