That's a false analogy. Taxi drivers are obligated to return lost property, but nobody is obligated to report bugs. That's why you create an incentive to report, i.e., the bug bounty.
Aye! Its not a perfect analogy but I was pointing out why people should reward the guy if he didn't exploit the situation in a wrong way. In this case, it was the whole source available to him. Albeit, he was more or less inclined to report the bug but what if he hadn't and probably sold it somewhere? why shouldn't the company reward for his effort.