Which is Crueler?

Not that I really agree with it, but maybe it's to stop cat fights?

If that were the case why would it be mandatory for single cat households?

Apartments claim that it's either to
A) Protect furniture or carpeting
B) To protect visitors

But both of those are absurd. Carpeting naturally wears, cats can be trained not to use carpeting or furniture, and there are other methods like SoftPaws and trimming.

Protecting visitors can be accomplished by requiring renter's insurance with the appropriate protection and making it the responsibility of the renter, which is anyhow. SoftPaws can again prevent the majority of issues there as well.