I'm actually not sure it is shortsighted. At least if you use me as an example, I don't care about online multiplayer. At all. But I do periodically buy PSN games, and I like being able to download demos, and I like Rockband and would be pretty damn bummed if I couldn't buy songs for it. So the prospect of being cut off entirely actually acts as a fairly strong deterrent to modding my PS3.
Of course, Sony loses money by cutting me off from PSN, but if this policy deters me from modding my PS3, I will have to continue paying for software instead of pirating it. It could be that a lax policy that banned multiplayer, but not other PSN benefits, would encourage modding and pirating and in the long run they would lose more money than they lose by banning a few people from purchasing stuff through PSN.