Any luck with figuring this out? More than 2 years later, wp8.1, Microsoft podcast app, and cellular data continues to be used despite "only download over WiFi..." setting being enabled. Problem appears to show up when one or more podcasts experience difficulty downloading over WiFi (stalled, or continuously restarting - snapping back from 50% or so to 0% downloaded). This, despite the fact that I have "for limited data connectivity..." setting in "cellular + SIM" set to "don't use cellular data". There should be no way that the podcast app uses ANY cellular data for downloading and yet it clearly sometimes does, in large chunks.
I do have "allow streaming" enabled, but the usage noted above occurs when nothing being played, or already downloaded content being played.
I've kept the problem under control by turning off cellular data when I'm connected to WiFi (thank you, quick actions in pull-down notifications area!), but that's a pain insofar as I forget to do so, or forget to re-enable when I leave WiFi area.