Because there's a difference banning someone from a server and writing code specifically to target someone you don't like (while distributing that code on the discussion board that the mods/Square Enix themselves stickied at the top). Again, you think Valve and Square Enix would approve?
The implications of people targeting each other using SteamIDs to target people they don't like and take their toys away from each other is pretty dangerous. Especially within modding communities for stuff like Bethesda games and Arma.
We don't need more shitty behavior from modders. Trust me.
What we really don't need is your delusional entitlement. Valve and Square Enix in no way are endorsing this mod, this is a mod made by an outsider, there's plenty of links shared on Steam Forums from stuff the Valve doesn't endorse, so your point is moot.
He black listed the guys that spammed the forum threads and were endorsing piracy, so I simply don't care. He's not making a precedent, the tools to black list people using Steam API has been in there for a long time.
Btw Modders make performance mods to help themselves and others to get the game to simply just perform better, why do you think Modders suddenly want to literally start limiting who uses? And even so, would that be wrong? You have no right in saying what they should or not do, they're doing charity.
The only thing this whole situation could make a precedent is more modders could follow his example and restrict their mods to only legitimate users, and I bet that scares a lot of the people that were bashing him on the Steam forums.