JetBlue interestingly clearly states that scuba tanks are not permitted, empty or full, in either checked or carry on. . . .
Hmm. At least Jet Blue is clear about it.
I agree with the rest of your post. Indeed, how would the airline know there is a small tank in your checked bag (without valve, to satisfy TSA).
I think it's clear why, even though TSA permits it, Jet Blue prohibits a tank in a checked bag: to attempt to collect an extra fee from the passenger.