My experience with certain transporters/graziers is that they simply don't care enough to notice that one of the stock has delivered a premature calf.
Happened once, I didn't realise it till much later (treating an injured cow, didn't notice she was no longer in calf); that calf must have been *on* the truck. No way they could have not seen it, but they never said. Happened a second time, I visited the cattle and when I was "what the...!" the overseer tried to convince me it must have happened yesterday and they were just about to tell me except she was standing at the gate. It was obvious at first glance that she didn't slip the calf yesterday, more like a week before.
There's honest people out there who will do the right thing, just not enough of them.
Heck, I know a contractor who wouldn't employ a guy because he would 'plough through a water line and not tell the farmer'. This is no different.
In this country there's no guarantee can be given that a cow is pregnant, last time I had a group of cows scanned for sale the vet wrote "It is my opinion that these cows are pregnant". He's not allowed to state it as a fact. Because none of the PD methods are 100%. Some breeders will guarantee delivery of a live calf meaning, if anything happens they will replace or refund the animal regardless of the reason.
So even if the vet could prove that she never was pregnant there probably is no legal comeback for her having been sold as in-calf.