Actually, that is a load of rubbish. If they are declared Treasure Trove by the County Coroner, he will share the full commercial value of the coins with the landowner. Not only is this extremely fair, but it dissuades people who find these hoards from disappearing with what they find, cheating the landowner and robbing archeologists of the chance to properly analyze the find and put it in an historical context to learn more about history from this period. It is especially important to have a policy like this with coins of the late third century because Rome was in a state of chaos during this period and left relatively few surviving historical records and much of what we know about Britain in this period is derived from the coins that the Romans/RomanoBritish left behind...
Moreover, the value of the find is enhanced by the provenance given to it by the archaeologists.
A pile of roman coins in a jam jar is not worth that much. A properly retrieved trove is another matter.