» This is just one of those situations where something goes so strongly against my intuition that I can't imagine how it could possibly be correct.

I think there are a few things at play here

1. Principal agent problem https://en.wikipedia.org/wiki/Principal%E2%80%93agent_proble...

2. Illusory truth effect https://en.wikipedia.org/wiki/Illusory_truth_effect

So like you said in the beginning, it makes sense that people selling something will highlight "evidence" that it works (principal agent problem).

Marketing is counterintuitive though. Please take a look at illusory truth effect and tell me what you think.