IBM found out the hard way that exclusive tie-in was against the law. Once upon a time they tried to force only IBM brand punch cards to be used with their machines, but International Business Machines Corp. v. United States, 298 U.S. 131 (1936) put a stop to that.

So imagine that GM says "you can buy this car with remote start, but you can only use said feature if you also purchase a subscription to GMremote. And using DMCA protections we will block you from using an aftermarket remote starter.

That probably wouldn't fly.