Many people here are selling illegal or hacked accounts one by one so i don't really see why account generators are being banned since they are doing same thing just at a larger scale. Just look at digital goods section , tons of hacked stuff there.
The question isn't why account "generators" are banned, but why the single sales of hacked/carded/... accounts are allowed.
Most comes down to the question how to prove that the accounts are indeed hacked (as for account "generators", there the question is practically obsolete).
There's several threads discussing this, if you want to jump into.
As for account "generators", it's good they are being deleted and I report any thread related to them when I see one.