I don't know.
My family has hosted international students for the last 5 years or so. They're typically kids, 16 years old, going into Grade 11. We've had kids from Spain, Germany and Japan. It's not a way of making money, but it at least covers our costs.
SInce we're also a hockey family I also reached out to the WHL Oil Kings one day. They didn't rpely right away, but when they did they mentioned how the program worked. But what surprised me was how little they paid. For 19-20 year olds (we were in the wrong part of town for 16-18 year olds as they'd go to a different high school) it was like a third less for room and board then we get - but there's access to free hockey tickets and the like.
So obviously you can't characterize the motives of all of the thousands of billet families out there, but if my kid was proposing to give live with a billet family somewhere that would be me huge concern: that they're more concerned with the hockey side of things.