Right, but I think it wasn't a real intended use case and that some TVs provided amperage over the spec (maybe by accident? simpler circuit bridging the same power pin for USB and HDMI?).
I had the same first gen Chromecast (may even have it lying around somewhere) but it came with explicit directions to use the included power cable, so maybe they updated the included guide some time after release.
They probably just provide extra power over the port. It costs extra to design an extra supply for a specific port so it’s probably shared, and likewise also costs extra to current limit each port. So more than likely a cost saving measure
I had the same first gen Chromecast (may even have it lying around somewhere) but it came with explicit directions to use the included power cable, so maybe they updated the included guide some time after release.