It wasn't. Without "!" it isn't obvious that you cannot take a:

  let a: Result<u8, u8> = /* whatever *;
  let b = match a {
      Ok(v) => Ok(foo(v)),
      e => e,
  };

and transform it into:

  let a: Result<u8, u8> = /* whatever *;
  let b = a.map(foo);

if "foo" is a macro. When you substitute "foo!" for all the "foo" above, it is obvious.