Now when you go permission hunting and you knock on a farmers door and the farmer says 'yes', do you ask the farmer if he owns the land, or do you skip away happily because you have a new permission. At a rough guess I would say 80% of people would skip away happily, delighted with their new permission.
Of course, the right thing to do is to ask the farmer right out if he is the land owner or a tennant. If he is the land owner, all well and good, but if he is a tennant, then a further search is needed to track down the land owner. But how many detectorists would do this, a 'yes' from the farmer is enough for most, after all, we know how hard it is to get permissions and hearing that 'yes' is sweet music to the ears.
So what is the problem of detecting a tenant farmers land?...Well, just say the land owner made a visit to the tennat farmer and driving up the track he spots you detecting away. If that land owner is against detecting on his land its possible he could throw the tenant farmer off his land, as well as yourself.
Another thing to think about is what if you found a hoard and had to declare it as treasure trove. Its quite possible you would lose it because you did not have permission to be there from the land owner. Also the insurance you may have would be invalid. I expect there are a few other reasons I can't think of.
At the end of the day, for peace of mind, make sure you have the permission from the actual land owner before you venture onto the land.