I'd buy your argument if Google identified up front that by accepting the $50 Play credit you could receive your phones after those who order it when it was released but since they didn't do that the premise is that it is first come, first served. And, I would bet most people who ordered early didn't do so because of the $50 Play Credit. They did it because they wanted their phones ASAP.
I agree with you on that assumption except for the conclusion, that is, what if Google didn't expect the phones to be available when they were and as a result, the outcome of first come first served, got all messed up because they were able to start taking orders earlier than expected.