One thing's for sure - Amazon wouldn't buy a company if they weren't going to turn a profit from it. And that money's got to come from somewhere, and some of it is going to (I assume) come from the developer's pocket.
There are various assumptions here. For one thing, sometimes companies don't acquire something to turn a profit in that area but as a marketing exercise to boost a different area. Or they actually shut down the acquired company to remove competition (although in this case Amazon doesn't have a games division so there's no competition) But yes, I agree that generally acquiring is done to make a profit. Also the money doesn't have to come from the developer, especially if they want to keep developers coming to them. They could just use their marketing expertise and financial clout to make Reflexive a much bigger player and make their profit that way. It doesn't have to be bad for developers. Naturally I say this all with the caveat of "we'll see" ;-)