As mentioned, the FAs are not security guards. I'm sure that physical restraint is not within their job description, and I don't find any fault with that. But police and such are charged with such duties, and the threat/promise of such is what the flight staff have at their disposal. So, they may not be able to contain the behavior now, but one hopes that the promised alternatives have that effect, as in the case cited by newyorkgeorge.
Cheers.