The User's Guide says:
The only thing that differs between operators in types and
operators in expressions is that ordinary non-constructor
operators, such as + and * are not allowed in types. Reason:
the uniform thing to do would be to make them type variables,
but that's not very useful. A less uniform but more useful thing
would be to allow them to be type constructors. But that gives
trouble in export lists. So for now we just exclude them.
Conal has pointed out that the uniform thing would be useful for
general arrow combinators:
liftA2 :: Arrow (~>) =>
(a -> b -> c) -> (e ~> a) -> (e ~> b) -> (e ~> c)
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