AGPL doesn't fix the problem as it isn't a matter of these companies using the code, but the developers being taken advantage of unfairly. Amazon publishing their modified Linux wont be of much help to anyone who isn't Amazon and uses Amazon's hardware - the only one who might be helped is other entities like Amazon (namely, Google, Apple, etc). The problem isn't even companies not contributing back changes (Apple does, for example), it is merely developers essentially working for multibillion companies "for free".

Of course this is a problem that developers made for themselves and something that they were warned against but chose to ignore.