If they are real players then I don't think the bonus is so high.
If they are cheating it is sometimes hard to tell of course (and maybe not always black and white). When we doubt we naturally give benefit of the doubt, but if they cheat on big schale they will get caught. We often catch cheaters within the hour because of automatic checks, but even if we don't catch them before they finish the tutorial, those credits will be automatically subtracted again (if they were already spent the user can go into negative credits.)
I hope it's not necessary to explain this, but we think it's necessary for a game to be clean to be able to have fun on it, so we have serious multi-checks (both automatic and manual).