I'm one of those that agree with that claim, I've said something similar a couple of times during the last few years on this forum (I remember that once I even used the term "golden handcuffs" in order to describe the whole situation).

As to why and how this came to happen in relation to the free market? The short answer is that both Google and FB are de-facto monopolies. In a way that can also be extended to Apple and MS. Of course that these companies will make tons and tons of a money during a period when software is eating the world (I know it sounds marketing-ish, but it's the reality). As such, they can use that money to "park" the best developers available among their ranks, so that no real competitor can emerge.