FWIW, it's not my method...it's one of those listed in the TCNA handbook, which is the industry guidelines. Some suggest leaving small gaps for any moisture to escape. But, so many people think tile is waterproof, it gets messy. With today's porcelain tile, they nearly are, though. Some studies have shown 70-80% of tiled showers are not done to industry standards, things get a bad rap. It's not particularly hard technically, but all it takes is one omission or error, and things can fail...you must have good workmanship and understanding...something sorely missing in the industry except for much too small of a minority.
Basically, if you use waterproofing on top of the backer, it must be complete up to at least the top of the shower head. If you opt to do a moisture barrier behind it, it must lap into the tub over the tiling flange. Anything else and you risk moisture getting into the structure and rot. So many people don't level the tub, and that can allow stuff to pool and create problems, too. It all works if you do it 'right'.
Personally, I like a profile at that junction...no caulk to fail, and it can breath a bit. Same in the corners. Per industry standards, all changes of plane or materials needs an expansion joint. The most common is caulk which can fail, followed by an actual gap, which in a shower isn't a good idea!