Really? You'd think at Amazon's scale an additional PSU in a 1U custom-built server (I assume they're custom) would be a few tens of $ at most.

Actually, now that I type that it makes sense. Scaling a few tens of dollars to a bajillion servers on the off-chance that you get an inbound power failure (quite rare I'd reckon) might cost more than what they'd lose if it does actually fail.

So yeah, they're potentially just balancing the risk here and minimising cost on the hardware.

Edit: changed grammar a bit.