I might want to walk an extra three miles, or it might start raining and I might want to put up my tent right here. Or maybe over there. Secondly, trying to find out who owns the land, then who to seek permission from just seems to be more trouble than it's worth. I've never had any problems because usually I camp somewhere really remote. Obviously one should avoid putting a tent up where there are 'no camping' signs, or on farmland or in someone's garden.
Discretion is very much the order of the day here.
I suppose the phrase 'out of sight, out of mind' sums the situation up best.