RE3 was a reverse engineered product. It used the original source and logic to derive an ABI compatible implementation. This has always been a grey area-outright illegal activity. It's the entire reason Compaq clean room engineered (or, at the very least, claimed to) their BIOS all the way back in 1982.

Every time I hear people complain about this, I just have to roll my eyes. Go do a clean room implementation like OpenRA and you're legally pretty safe. Use their IP and you're just asking for them to easily shut you down once you catch enough attention.