Because, IMO, functions take values (even if they are references or aliases), do something when called at runtime, and return zero or more values.

At runtime, It takes the name of a variable, allocates it* and returns the named variable as an lvalue.

* — The implementation differs, but this is the intent of my and it should be the perceived effect.

return isn't a function either

It is a function. It's not a function in the functional programming sense — it has side-effects — but that's not relevant here.

More specifically, perlfunc documents named operators, and return is one.

If you're going to add colons, why not semi-colons? Braces?

Semicolons can't appear in expressions. As for braces, both constructs such as do {} and the anon hash contructor *are* mentioned in perlop.