It could also be because enforcing non-griefing is a pain in the ass. Every, and I mean every, anti-griefing plugin has holes in it that the more cunning griefers take advantage of. Then you have to go to the trouble of finding whodunit using your block logging plugin(and hope your MySQL server wasn't down at the time. ). Lets not forget that many griefers have alts and proxies as well.
But I agree with Doivid. Griefing is terrible and wrecks the experience for anybody who just wants to build.