I think this trust (in the Post) is now misplaced, and in the case of the Post and Amazon, you absolutely shouldn't. But perhaps it always should have been with any single newspaper.

This is why whistleblowers now often work with two different organisations with different ownership/politics, or in different branches of media, or with a journalist backed by the ICIJ (e.g. the Mossack Fonseca leak investigation was shared with the ICIJ).

But yes, any generic online whistleblowing broker with dozens of concurrent cases is going to be such an obvious target for state or organised crime interference. Anyone making a business of brokering whistleblowing for a cut of the reward is an obvious risk.