Game mechanics are not considered copyrightable[1]. If you had a clean room implementation with your own significantly different assets, it would be allowed.

However, the exact definitions of "significantly different" and "assets" is where things start to get fuzzy. While you could definitely make a very similar RTS game, exactly how similar can you get? EA doesn't own "military-themed RTS", but they probably do own "Soviets vs Allies with about 5 different unit types, air transports, and tesla coils." Getting even more fuzzy, are unit abilities considered assets, or game mechanics? It'd have to be worked out in court.

My gut feeling is these clone engines would probably lose in court. I think the specific expression of the general game mechanics being cloned here probably would constitute infringement. But there isn't much upside to the IP owners to pursue enthusiastic hobbyists cloning a 20+ year old game in a non-commercial way, so they let it slide.

[1] "Although Amusement World admitted that they appropriated Atari's idea, the court determined that this was not prohibited, because copyright only protects the specific expression of an idea, not the idea itself." https://en.wikipedia.org/wiki/Atari,_Inc._v._Amusement_World...

I'm sure if EA could undo their release of Red Alert and C&C as open-source, they would.

OpenRA simply downloads a copy that it loads for the purpose of assets, but the engine is completely new, and it is very different from the orignal Red Alert. At this point, I don't think a single unit acts exactly the way it did in the original game. It's endlessly being rebalanced.