Because I believe in multilayered security. No one approach to security is sufficient, but combining as many approaches as possible can allow each approach to help cover the weaknesses of the other approaches.
Also, I disagree that denying the lookup of certain domain names is abusing DNS. If I were running a DNS server that was being used by the public, or that was being used by downstream DNS servers, that would be different.
Also, I'm not aware of a method that can accomplish the sort of coverage that blocking DNS lookups can. If you have an alternative, I'd be genuinely interested in hearing about it.