I spent an hour on the phone with support, and then senior support staff today.
Senior support actually attempted to sell this to me as a mirroring feature of iCloud Music Library, not a bug.
Music.app is currently attempting to download 5,615 songs to my 64GB iPhone 5S and generally crashes when I try to stop all downloads. I asked how is there not a toggle for this? It's slowing my 5S to a crawl, and I am repeatedly running out of storage. Unbelievably, she suggested I turn off iCloud Music Library all together. I said the whole point of signing up for Music was the ability to access my lifetime purchased library on my portable devices from anywhere (and being able to stream what I don't own) -- not having some aribtrarily-chosen sliver of my 1TB library on a 64GB device! (Is anyone else having incomplete phantom downloads still show up when you choose "Only Downloaded Music"?)
At the very least there should be two Cellular Data enabler toggles -- one for Music streaming, and another for iML/iTunes Match downloads. It's ridiculous that I can't listen to streaming Music without my phone filling up with unsolicited music on my data plan dime.
I suppose at this point I am just hoping and praying the iDevice downloads will stop once my primary iTunes library Mac stops uploading match files.
Grr.