The thing is, it means datable evidence is taken out of context and makes putting a rough date to a site extemely difficult. Especially if it is a complex site. Also damaging other pieces of evidence along with it.
There are some 30 acres of land which my family farm with some interesting archaeology and we ourselves have been told not to go digging around and to notify the local archaological society if we find anything, which at times we have done. Last year the plough uprooted a carved 3 foot long limestone window sill, which vanished over night before the archaeologists could record it and take it away to be preserved. Clearly some one thought it might look nice on their rockery and not in a museum where in my view it belonged.
To top it all off a bloke who we have cut hay for a few times asked if he could go metal detecting in that particular field. To which we said no you can't, his reply to this was " but I want to try and find something more valuable this time" So not only has he admitted selling an artefact which doesn't belong to himself, he also did it without our permission and the supervision and advice of a professional archaeologist. Along with that people are getting into an annoying habit of walking their dogs through the fields and get very aggressive when you tell them there is no public footpath through the field.
It's sickening how people today behave in a way that only they themselves matter and that they can do what they want as they see they are doing no harm, when in fact they are.