Niknak, we'll agree to disagree.
No, it isn't crazy and it's no different from asking any other transport operator of any sort to make a one way journey
Surely the VAST majority of taxi journeys are one way? Furthermore, many are not allowed to randomly pick people up off the street.
I can get a taxi from the centre of Derry in Northern Ireland to Derry Airport, about the same distance as St David's to Exeter airport, 7 miles or so, for about £13. Fuel costs in Northern Ireland are MUCH higher than in England, as is insurance, so the excuse given by the fine taxi drivers of Exeter doesn't rub I'm afraid, and nowhere else have I heard such an excuse.
For comparison, a taxi from Bristol airport to Temple Meads recently cost me £28, only a few quid more than the trip to Exeter airport. Despite the driver not ending up in the middle of nowhere (as is argued by Exeter taxi drivers), he was not allowed to pick-up a fare at the train station.