The same thing is in the terms and conditions for each of your old CD ROM games. The point is that they can't physically keep you from using the DRM free software that you backed up locally.

The perceived difference has nothing to do with the game being a "service" or that perpetual licenses are not economically possible for "services" but with the fact that by the power of the Internet companies now have a way to brick your stuff remotely. And you accepted it when they put it in instead of voting with your wallet. Because you wanted Half Life 2 just so so so badly.

They're doing it because they can, not because they have to.