What I think is that just instead of this service giving as free or something like that if they started taking a fee for that than you will have to pay anyways . So instead of that if say the tip provided say is correct so you tip it and if it's not correct you do not tip it . So only on profitable bets you can tip it .
That's the thing there and likely the reason people are sharing tips. Some of them share tips for free for a time and then ask for a smallnpayment the next time. If they become popular in providing good tips, then of course their charges would get higher. The more people looking for tip from them, the more it will be profitable.