[oskarth]

Presumably the goal of the bounty was to make Prezi more secure. OP found a serious security hole, without using a "violent" approach (spear phishing, cutting the power, etc). OP reported this security hole.

In a legal sense, they aren't obligated to pay. There are a lot of legal loop holes. By not paying for something that they obviously want to know, they are discouraging other security researchers to disclose "out of scope" holes. To what end?

If you succeed, we will give you cash. That’s right; we’ll pay cold hard currency into your bank account. Think of it as a thank you. (Prezi bug bounty site)

I guess the right way to read this is as a (legal, of course) fuck you.