Because by making it a regular fs you immediately get support in every piece of software.
Every file chooser on your system suddenly knows about tags and gives you a way to query them.
Basically what Spivak said. I was interested in adapting an existing interface (a filesystem) to do the set intersections because it means more software could benefit from it without additional work.
I must say honestly i'm not so fond of using ordered sequences (posix paths) in place of the unordered sets which they should be. I guess my question is really why you wanted to make a fuse fs out of it (versus say some tag-based filtering script like notmuch). (just for fun is surely a valid answer)