I’ve used the relocation facility of a rental car company in NZ a fair bit. I only used it with a specific company, I didn’t use the service you mention. In all cases it was not fuel paid for and there was a limit on days and kilometers allowed. I find it most useful if I’m travelling via rental from A to B anyway but I get a way to avoid the car rental cost itself.
It is annoying though to see a price and then find all the ‘optional’ extras later as you note.