Because the cost of determining who is good and who is not has a great
cost. If you buy an IP block, regardless of your intent, that IP block
should not have the ill-will of the previous owner passed on with it.
Might as well be the end of discussion, right there, then, because what
you're suggesting suggests no grasp of the real world.
the previous owner sucked, the new owner should have the chance to use
that IP block without restriction until they prove that they suck, at
which point it will be blocked again. That system seems to work well
enough: blacklist blocks when they start do be evil, according to your own
(you being the neteng in charge) definition of evil.
What you just described doesn't implement what you claim, at all.