since downloading endles amounts of copyrighted material via torrent or similar is still illegal whichever way you want to slice it, the companies can argue that it's a fair limit since many users who dont go a bundle on downloading pirated material will go nowhere near the 10gb limit.
as I said thougth, it's not like they'll charge you for it anyway, just turn the speed.
I think I'll put a counter on my router and see what my monthly amount actually is, but between a laptop and a PS3 I doubt I'll be troubling servers too much