[droithomme]

One can reasonably argue that if Apple has a bounty program and therefore finds information about critical security vulnerabilities in their products valuable, that one should prefer to sell information about such to them.

However the situation is that they do not and thus the absolute economic fact is that Apple considers such information utterly without value. Given these there are no obligations upon hard working security researchers and they are free to sell to someone who does find such information at the least trivially valuable. In fact it would be utterly unethical to do otherwise. A man is worthy of compensation for his labor provided that labor has value. If one party finds his labor of no value that is not a problem. If some other party, such as the NSA, finds it valuable then they have the right to sell it.

Obviously there are (unethical and disingenuous) trolls who will bring up scenarios where things are illegal. We are not considering those. Implicit is that this is an economic transaction. Which can consider ethics, such as the right to be paid for valuable labor. But does not extend to the right to commit crimes. The constraint here, obvious, is that we are discussing legal commercial transactions for legally performed work.