A title like that probably will attract the moderators quickly

I don't have any intention of doing so any time soon, but was curious about this. Clearly, "Morroblivion" is copyright infringement, because of gamesas's stipulation that official Morrowind content cannot be ported to another game - not even another one within the TES series.
Does that moratorium also extend to the other TES games - Arena, Daggerfall, Battlespire, Redguard, etc.? I would assume that it does but have never heard anyone address the topic.

And, as a follow-up question: why are unofficial Oblivion mods that port limited content from one of these games ("Morrowind Books", etc.) not considered to be in violation of the copyright?

I would think it applies to all games in the franchise as well as all games by other developers -- Main reason they do this is that all game companies purchase material from third parties in the process of making a game and for the most part the license they purchase is for using that material in the one specific game thus reuse would violate the license agreement between Beth and the third party and open them up to litigation if they allowed it (sure the modder that used it would also be open to litigation but who do you think the asset maker would sue the individual that made a mod or The large corporation that sold millions of games that allowed them to be imported to their game ! ) -- and since it is impossible to list every asset and where it came from and which is reuseable in other games it is much easier for them to just not allow it at all.

As far as the small mods that port limited content - if they are directly taking material and simply importing it to the new game it is in violation and if they are notified of it they will remove them just as they did with Morroblivion but if they take the general concept and remake the content then it is usually allowed (ie. remakes of morrowind armor that is made for Oblivion etc. ) -- The books Are a bit of an exception since evidently Bethesda created their content and they have pretty much allowed the content to be reproduced as a part of the lore.