Well, they don't need to spend money to schedule my Lambda, but they did already spend a bunch of money building out the datacenter so they can pick up my 100th of a nickel when they do schedule it.

AWS is kind of like a bank, making money on the spread between the cost of compute at different time scales. They can get cheap compute (by committing to owning datacenters and filling them with silicon) and sell expensive compute by letting customers buy it by the second. This is a great business if the demand for per-second compute is always rising, but it isn't if all your customers suddenly take you up on it all at once and start spending drastically less but your huge datacenter investment is still illiquid.