Well for starters he's not the one who made the game so he shouldn't be policing something like this.
His mod, his rules. You can play the game w/o it if you want.
Then comes the time when it stops working on steam copies because for some reason the steam_api.dll gets updated and then it stops working. Then what about the people who decide to crack their games after purchasing to remove needing to be online to run the game. (Steam will sometimes bug out and make you go online to play it at least once in a while). When he discontinues it, and it breaks why would people need to go through the extra effort to fix something which shouldn't have been a problem in the first place.
Assuming he does stop supporting the mod at that point, why is it such a huge deal for people who think the mod is important to continue to update and support it if they feel it has value? What if the mod stops working for any other reason and he doesn't feel like updating it? It's open source so people can look at the code and do whatever to it. I don't think he's even that concerned with locking pirates out forever and ever, since posting the code like that encourages people to make alterations. But most of the pirates playing this won't want to jump through that hoop and will sit around until someone else does it.
It's a dude making a mod in his spare time for no compensation. If he could magically check whether you were wearing a shirt that was his favorite color first and make the mod not work based on that, I'd think it's weird as fuck and probably not use his mod unaltered but I wouldn't say he's violating some ethic principle by doing so.
Edit: It's even been altered to work with cracked versions
already so this line of argument is a lot of to-do about not much. It's apparently not that hard at all.