Restricting symbols to a strict subset, rather than a strict superset, of global keyboards in practice, is about cultural literacy in practice.
If you put unicode in variables, perhaps because you're using a national keyboard with special unicode powers, your code will be extremely hard to work with for programmers using a different national keyboard. Thus, even if you could do it, you shouldn't.
Now, the symbols on American programmers' keyboards are not all on every keyboard in the world - but pretty much every programmer in the world knows how to find them. Thus, sticking with ASCII is basically sensible use of Postel's Law in the language design context.
In comments - it should be ok but I think it breaks right now. Not a high priority bug, but definitely a bug.