The more you spend the more credits AWS gives you. Big spenders may get $200k of free credits while most don't get more than $15k. You can opt-out of CreditHero anytime and use those extra credits however you like.
Surprisingly, yes. AWS Promotional Credit Terms & Conditions prohibit re-selling, but nothing stops one from hosting friendly startup's apps in their account for free. In fact that's a common practice among founders.