It is fine, but ...
... most people don't do it.
A programming language is defined by its implementation so if the implementation has a way of calling shell commands then the language does. In terms of our existing policy these are clearly already allowed.
This is also, in my opinion, the most sensible policy. We can already see that there is not a huge flood answers using this trick. When left to their own devices members of our site choose not to do this most of the time. So I don't see any benefit in banning it. And if we did ban it we would get into the usual fuzzy edge case deliberation and the definition of what really constitutes this trick would have to be complex. A lot of worth for no or negative gain.
So why don't users do this already?
Well here I have to speculate, but I think it has to do with out site culture. We really value the cleverness or skill of the golf perhaps more than we actually value short code. I golf in Haskell because I like Haskell and I want to golf in Haskell.
Using the shell and a generic wrapper defeats this purpose, I might as well just golf in the shell anyway.
I think that it not being very fun to do is a strong enough incentive structure for us.