There are two security cameras in my current place (though both inactive). The person I replaced for my roommates had a falling out with them and trashed the place when they wouldn't let him break the lease(a lease he insisted on being longer).
So they had one in the kitchen that basically captures the kitchen and livingroom all the way to front door, and another pointed at my door in the hallway.
Two places before that, the house owner had one for the main entrance inside, a few more for kitchen and livingroom inside, and at least 2 outside.
It's pretty normal for some people.
I also worked with someone years ago who was paranoid about home invasions and didn't like the people his girlfriend's daughters were seeing, claiming they were gang members etc. He showed me his feed to see inside his own house when he was at work.
So tl;dr: lots of plausible reasons, why do you find it so hard to believe?