It's actively crippling the competition. They're not competing with Sony; they're just trying to ensure Sony can't leverage the advantages of their own platform.
That's not being competitive. That's being anti-competitive. Competing means running as fast as you can. It doesn't mean coating the fast guy's shoes with lard because "it's not fair that he's faster than you."
No. The reason why larding up someone else's shoes in your example is "anti-competitive", is because it's against the rules. That's why it's wrong. But if we had a sport where that was an allowed skill, then it wouldn't be anti-competitive, it'd simply be scathingly competitive.
Microsoft didn't break any rule. The only court that will judge them is the court of public opinion. We may not like their way of doing business, but to call it anti-competitive really isn't fair.
Examples of actual anti-competitive behavior include agreeing to price fix with rivals, predatory pricing, tying, price gouging, and refusal to deal. Anti-competitive behavior, in other words, is manipulating consumers and free trade for marketshare. It doesn't include parity contracts, covenants not to compete, protecting trade secrets, and contracting for exclusive control over intellectual property.
It means behavior that actively curtails market freedom by limiting consumer choice and creating barriers to trade. And again, that does not include parity contracts. If it did, then many other contracts which restrict workflow would also be "anti-competitive".
In other words: to say that it's not in the spirit of competition to throw your weight around is actually asking a corporation to try and compete with their arms tied behind their backs. It may suck. But corporations should be allowed to play hardball with their contracts in the
spirit of competition. It may subject them to criticism, as the parity clause rightfully did in Microsoft's case, but it's really not anti-competitive in any sense.