Whether it be to for the purpose of sale or for the purpose of separating the copy of Batman #1 from the rest of the pack, I can see a number of reasons for doing so. While bound-volumes are cool, and often a cool piece of history, you can't enjoy them the same way you could a slabbed book. Frequent handling (flipping through pages) is not "good" for paper aged over seven decades. But even beyond the preservation factors, slabbing the book individually is great for presentation.
From a content standpoint, Batman #1 is very different from subsequent issues. The psychological depth and darkness of the Joker is greatly reduced by the time of Batman #2 -- let alone following issues. So beyond the obvious appeal of it being a "No. 1" with historical first appearances, Batman #1 is simply different in both tone and content from the rest of the run.
Hence, there is good reason to remove it from a bound-volume to appreciate it individually. (thumbs u
Content wise you are correct but considering that every issue from that bound volume is in that auction I don't believe the seller's intention in butchering a perfectly fine and well preserved bound volume was quite so well intentioned . And to make matters worse, the books don't even present nicely in slab. Yech.
-J.