Why aren't (free) podcasts distributed via Bittorrent?

Why don't we distribute podcasts via Bittorrent? I started thinking about this after hearing Ira Glass' latest pledge solicitation on This American Life (which apparently costs WBEZ $170k/yr in bandwidth). I don't mean some tracker where powerusers go and download torrents and manage ratios and all that traditional "bittorrent" stuff. I mean WoW. WoW is a Bittorrent client. If you play WoW, you're automatically seeding the updates and patches you've downloaded to everyone else who plays WoW. That's how Blizzard can distribute large updates with maximal rapidity and minimal impact/downtime.

Imagine if iTunes (or podcast app du jour) did the same thing. If you download a podcast, you seed it. Instead of Chicago's public radio station eating the six figure cost of my listening to their free content from Portland, OR, we all just get to upload 70 or 100 MB without even knowing, no harm done.

So, uh, is this a good idea? I feel like it should have been attempted by now....