The subsidy recipient is not necessarily the owner e.g. following mutual agreement a farmer could graze their sheep on the neighbour's land which they don't own (because friendliness or usefulness or family history or specific allowances when the land was sold a hundred years back), and that land could be unregistered.

I expect the idea here is that the subsidy seeker probably has an idea as to whose land it is they're using, and thus could go ping the owner for registration so they can get their subsidies back. This is especially likely if the registration is easy and free: chances are the owner simply has not bothered registering the land because they have no need to and can't be arsed, being asked to by an acquaintance would be sufficient a motivation to eventually check in with the local authorities.