Legality not as in breaking the law but rather breaking the agreement we've entered when we signed up for online content and multiplayer. Not sure if you mean that but used in that sense the term 'illegal' is correct in this situation.
In the end of the day I could care less about people flaming about hybrids though, I had my wingless Lamborghini Murciélago LP670-4 SV with Zonda C12 S 7.3 engine swapped into it in 2.11, and now in 2.12 it got its wing back and gained 150bhp to reach stock LP670-4 SV power figures while keeping the Zonda sound, and I still cruise it around in public lobbies with friends who enjoy the cars for what they are just like me.
It's these grey zones (just think of the drifting community) where you can't just outright blame them for cheating or doing something bad when all we do is improving the game for us without hurting anyone (as in giving the car a better sound and look that we enjoy more). We're the people that mostly got hurt by 2.12, PD took away this new joy from us, while leaving the kids with their 5 billion HP cars and the cheaters with their hidden upgrades, tyres (racing super softs are even displayed as the car's stock tyres in lobbies now) and all the other crap stuff that came with save file modding in the game. So while it's still a game, yes we do have a reason to be angry.