This has always been a thing, far far before the “latest in IaaS architecture”. The idle polling still exists except they manage it for you. Their margin is then improved if they intelligently reduce wasted polls through a variety of techniques.
However, yes, you don’t pay for this or care about it much. It’s good.
This is no longer the case using the latest in serverless IaaS architecture, where the pool of queue workers dynamically expand to meet demand.