I think the reason would somehow be related to the fact that if that becomes common practise, most people would just wait the couple of weaks / months. Resulting in the lack of the sales boom they are trying to secure.
I do think that something like that would work though, if they release a patch that rids the protection, after about a year. by then sales have dropped down to a trickle, and the effect on future markets should be positive, since it would generate a new boost in their sales for a short time (though i suspect this would be minimal, the sales on the pirate market would be boosted a lot rather than the sales that they can get royalties from).
