On its surface banning harmful things sounds like a good idea, and it is always possible to think of things everyone will concede has to be at least very strictly regulated.

However, this requires restricting personal freedom, hiring police and judges and prison guards.  It also requires spying on the public, putting innocent people in jail, or maybe just naive kids and thereby ruining their futures.

In short although it seems bans are the first reaction to harmful things, it should in fact be an absolute last resort in any society that calls itself free.