PTC Sites TOS-Terms of Service
Anyway, it’s their website and if you want to join, sooner or later you will have to respect their rules.
You must be careful because in the TOS is mentioned something about inactivity. If you don’t have activity in your account more than 2-3 weeks, the account will be suspended, and the worst part is that you can’t make another account from the same IP address. If you do not click your ads in one day, on the next day you will not receive the share from your referrals earnings.
By the way, I saw on another PTC something like this: if you do not invest in your account you will not be eligible of earning more than $10… meaning you will have to upgrade to paid membership or buy/rent referrals if you want to earn more than the amount specified.
I also found rules like: “you cannot earn more than 2x or 3x the amount you’ve invested in the ptc website” or ”you cannot bring more than 100 referrals”.
In the end I would say – read carefully the TOS of every ptc site you consider joining.