That is not the only reason people lease. Some do to see if they really want to get into ownership, others don't feel they have 365/24/7 to offer a horse, etc. Leasing can end up being more expensive than owning out right.
As far as the leasor complaining - it completely depends on how the lease is written and the history. I have a gal leasing from one of my boarders. Boarder is very very VERY absentee. If she decided to start coming around right now, even I would complain after all the gal leasing has done for that horse.
Not everything is black and white.
Oh no I completely know that people lease for other reasons, heck I did for 10 years because I was in school..not because I couldn't afford it. I was simply quoting what my trainer told me her view on it was.
But regardless if the owner comes around a lot or once in a while, it is still their horse and ultimately their decision. Yes common courtesy is nice, but sometimes not so common haha.