I'd say its just market forces.
Companies that are big enough and ugly enough to really dominate their spaces (LinkedIn definitely fits that description) get to call the shots.
APIs are one of the tools you use to get to that fortunate position - but once you're there, its time to pull up the drawbridge, dig out the moat a bit more, and pour boiling oil down on the heads of the wailing savages milling below.
The risks of course are that you leave yourself open to new entrants, who *do* play nicely with the developer community. But for a company like LinkedIn, where the network effect is enormously strong, that's probably not much of a risk - or at least, you'll have time to react and counter it before it becomes a significant threat.
Ironically the more (justifiably) exuberant we all become about the power of APIs, the more threatened encumbents might feel, and the more likely they become to want to lock down this stuff while they still can.