I had exactly the same issue, iTunes was downloading tonnes of tunes from iTunes Match via the Verizon LTE network, even though I had set the settings to not download anything from iTunes over the mobile network. I spoke to a senior tech supervisor (Michael) who said he would elevate this to the engineers - he's going to call me back to let me know the outcome. I will post an update here once I've spoken to him again.
In the meantime, a temporary workaround is either to turn off the mobile data completely (not helpful I know) or just to sign out of your Apple ID. Guess you could alternatively just go to the iTunes download section and manually delete all the pending downloads one-by-one although this could take a while.
Hope this helps...