if they return to a landowner what they've taken from them...
Let us take a look at a random page of the 'identification and valuation' page of The Searcher, to get an idea of the average value placed by the market on run-of-the-mill artefacts: £50, £180-200, £80, £40-60, £30, £120, £120-150, £220, 300+, £15-18, £60, £120-150, £80-90, £40, £120, £20, £30, £80.... I think we get the idea. Metal detectorists claim they are 'not interested in the money', but every single one of them is aware that the artefacts they rmove from a landowner's property are not without (monetary) value, and cumulatively the resale value of even a small collection of historical metal artefacts can be quite substantial. How much of the money raised by the sale of such a collection (for exaple by heirs) ever gets back to the landowner? How many artefact hunters create the sort of documentation that would allow this?
No. it is not the artefact hunter who is generous, these people are exploiting the lack of awareness of the landowners and it should be the job of the PAS to inform the landowner of this as a public service to STOP the rip-off merchant collectors profiting from keeping landowners in the dark.