That's not cheating, it's a agreed upon rules. Making an AI that can beat a human is already very hard. Next you'll add other arbitrary requirements, like limiting the wattage to what the human brain uses...
Machines are obviously better at everything requiring fast-twitch reflexes and massive data analysis. There's nothing interesting in a competition there.
In Starcraft, the fear is that machine-level micro may make it completely pointless. A human could play strategically better and still lose because the physics of moving a mouse are different than the physics of a transistor.
Controlling the motors of a drone or the moving surfaces of a fly-by-wire plane is much more complex than moving a mouse or hitting some keys.
Forcing an AI to play with a mouse, keyboard, and monitor is a huge handicap compared to giving it a God's-eye view API.
Taken to the limit, an AI with an API to the game can take micro-control of every character. His high-level strategy might not even matter, if every character can fight optimally. At the very least, an AI would have an advantage when starting multiple battles, since he doesn't have the "switching" overhead of panning around the screen, clicking on items to issue orders, etc. But that "god mode" option is not available to a human. (Even if we slowed down the game, the human would get likely get bored before playing optimally.)
But if the AI were limited in the same way a human was, the AI couldn't monitor an infinite number of battles at the same time, couldn't issue control instructions infinitely fast, etc.
Maybe the AI can click around "fast enough" that it wouldn't matter (we don't know). But at least it would be a fair fight.
This is different from checkers or chess or go. The Starcraft UI is part of the balance of the game itself, so if you change it you are changing the game. People do this all the time to help humans beat other humans, and we call it cheating. Why is it magically not cheating if we do it for AI?
If they do have to change the UI (e.g. hook a datastream directly into the AI), the opponent should be able to install any mods they want. Otherwise the AI is playing one game while the human is playing another.
If the AIs were to become strong enough to compete against Starcraft players, they would have the unfair advantage of being able to give a lot more actions per second.
[1] https://www.reddit.com/r/starcraft/comments/16mkmg/injuries_...