Isn't he just trying to call it a pyramid game without actually calling it a pyramid game? (or ponzi scheme, etc etc same same). I don't know the nuances of his position but maybe him calling it what it is (ie a scam) can get them into all kinds of legal trouble if they don't have hard proof. And because the fraudsters are as distributed as the ledger, it's hard to come by hard proof.
I mean, I think that Jack Dorsey is banking hard on bitcoin because he knows he's near the top of the pyramid and he wants as many fools as possible beneath him, but there's absolutely no way to prove that he's not just a soft idiot.
Just guessing though, all I mean is "if we don't, people might lose their money!" would have applied to Bernie Madoff too.
It's hard to know what his true intentions are, perhaps it's altruistic or perhaps he has his own reasons for being bearish. If he has hard evidence, he should present it, if he doesn't, perhaps he could instead use his position to call for an investigation or inquiry.
If he wants to argue that Bitcoin is a pyramid game, that's a different argument, but I can't see many people taking him seriously.
Pieter's arguments are along the lines of a "Think of the Children"[1] style argument, or an appeal to consequences[2].
"We must ban cryptocurrency because if we don't, people might lose their money!"
[1]: https://en.wikipedia.org/wiki/Think_of_the_children
[2]: https://en.wikipedia.org/wiki/Appeal_to_consequences