Freenom questions

So I've heard Freenom makes money by waiting until one of your free domains gains enough traffic, then takes it back and puts ads on them.
How much traffic can you send to the page before you can expect your domain to be taken away?
If I pay for the domain do they still take it away?

Why not just buy the domain? Most places you can find them from 9 to 15 buck. Cut out a few drinks a month and the cost is covered then you don't have to worry about anyone taking it back