Well, I guess it is understandable to be anxious about that.
Some reasons that come to mind:
1. We do not know the details of the anti-bot techniques implemented by Apify, and we should not know them, of course, but that is simply a fact. We can only trust that they work well enough.
2. We do not have enough data to decide how to handle free users. We would need detailed per-user data to make better decisions in our actors.
3. As you pointed out yourself, if the incentive were higher, the system might not work because of sophisticated scammers. But those scammers could already exist now, just targeting specific actors. That is what we are afraid of.