If I had to guess, it's because he doesn't want to sue a company he has a stake in. That's like suing yourself. Patent and intellectual property copyright is a funny thing; the owner can enforce (or try to get it enforced) when they feel like it. They usually let it go when it's to their benefit.
Wonder if microsoft has a deal with Allen or if he just doesn't want to sue the company he has a stake in.
This could easily be spun as Allen as the attack dog against microsofts competition.