Oldcock, Hi, and belated welcome to the .Net!
I don't think anyone has suggested thgt the larger yards aren't providing full tanks, just that they seem keen that you only use their fuel... even this isn't a foregone.
If you hired a car, van or truck it would only make sense to fill it up at the cheapest garage before you returned it! That's the point being made.
Before they sold out to developers Topcraft did, in fact, have dipsticks attached to their fuel caps; so there was never a problem of how much fuel was onboard. It will be interesting to see if they're still there!
Both SallyB and Friday Girl do carry and use them!