Mod sites like the nexus and such are usually very strict about their usage policy, if you do not follow it to the letter any modder can ask for it be taken down if user was not asked for and permission given, if the user was not given credit properly etc etc. Whole list of guidlines on site about what you can and cannot do with other peoples mods.
I a, dubious about the copyright claim however, now i am no expert on copyright law, but how the hell can a user made mod posted for free on the internet, and available for anyone to download have a copyright exactly? Seems some mod makers just grabbed some legalese to toss in their rant letters to make it sound like he was in serious trouble and threaten dl sites.
For the people say he just grabbed a bunch of mods and reposted them, not exactly. Not playe mw but have played oblivion and fo3 and fonv with mods, and let me tell you adding a ton of mod to any bethesda game is playing with a loaded gun, their engine does not have a good way of dealing with multiple mods and conflicts, even if there are no serious conflicts to be found mods in general can make the games unstable and crash happy. even if you take the time to sort out the "proper" load orders as described by any number of mods and how to guides, you are still going to have a game that crashes more often than not.
So if he took a a ton of mods and got them all working together and stable he put a ton of time into doing this thing and he did not just collect a ton of mods and upload it as one big file.
And for a game as old as morrowwind, hell if anything interests people into getting the game and playing the game like this thing did i think the modders should lighten the hell up.