The saying goes, "hard work pays off later, but laziness often pays off now" Such is the way of the developer. Gimping ports will not motivate people, and if they think it will help sales. HA, that's a good one.
Who said anything about gimping ports to help sales? If anything, they're choosing to not even waste the resources necessary for a "gimped port" of that feature because they know the hypothetical attention multiplayer would supposedly bring isn't worth the time and effort...