Potzato: They take advantage of addictive properties/gameplay. This is a trap for some people, a very hurtful one.
Cormoran: You're thinking of something more along the lines of what free to play provides. With a monthly fee you get what you get for the fee, with free to play you're offered some exceedingly expensive options to take away the withdrawals they create.
No : with a free to play, even if you never buy anything, you can play. With a monthly fee, you bought a game, then you have to pay again each month just to keep the right to play. So, if you stop paying, you can't play anymore (no offline mode).
Would you agree to buy a book and to be permitted to keep it only if you pay something each month? No, I guess. That's the same with games.