This is a tragedy of the commons scenario that will ultimately lead to people being less friendly. Right now it's assumed that if someone comes up to ask you for something, they have a good reason.
Those three groups contribute substantially to what people expect when they meet a stranger in public and adding more disingenuous possibilities would make it worse.
In fact, I don't think you're quite right about the tacit assumption involved. I rather think it is assumed that if someone asks you something, they it is not for instrumental purposes. If someone is talking to me, it's because they want something; that request had better be legitimate. And this is what I find unnerving about "Rejection Therapy" and its ilk: I'm being used as a means to an end.
I think we need more interaction with no-strings-attached, not less.
The favors seem harmless, with examples from the article including asking to try someone's sunglasses or to meditate with someone. Activities like these are also a part of exposure therapy for people with anxiety and/or a speech disorder.
Separately, though this depends on your location, I already disbelieve that someone asking me for something has a good reason (which is a common attitude in dense cities). I immediately assume they're trying to pull a scam or sell me something, and most of the time, I'm correct. (A funny example of this attitude is one of the top posts on r/AskNYC, though I'm not based there [0].) If someone were to approach me just as an exercise to address a psychological issue, that would be a far better outcome than what I would expect.
[0] "Millionth Cyclist on Manhattan Bridge" https://old.reddit.com/r/AskNYC/comments/dk76kv/millionth_cy...