I would wager that, more than the costs of serving these API calls, preserving the opacity of the resultant network is probably part of the advantage these companies get from locking down their APIs. Given how much flak they already get for the mental and social damage done by social media and Twitter specifically, I suspect they're very happy to preserve as much of the black boxiness as they can so they can point to the value users get and their ad revenue and say that all the costs are unfortunate coincidents rather than central problems with the paradigm.