Blizzard have done this for WoW updates forever. MS may run into some liability for inducing data overages depending on how up front they are about it and how badly it bites people.
Are Blizzard up-front with their users about what is happening? The covert nature (or otherwise) of the uploads is a significant concern here. The average geek playing with Linux doesn't complain about BitTorrent also uploading the distro image they're downloading, but the average geek playing with Linux knows the deal and decided voluntarily to participate.
MS may run into some liability for inducing data overages depending on how up front they are about it and how badly it bites people.
I would expect that if the report is accurate and if this behaviour was activated without the user's knowledge or consent then Microsoft would be on the hook for any and all damage caused as a result. That could mean a small extra charge from an ISP for bandwidth over a cap. However, if the behaviour means someone's Internet connection didn't have enough capacity left to do something important that it otherwise would have, whether because the uploads themselves limited the spare bandwidth at the same time or perhaps because it triggered some sort of degraded service from the ISP after going over a cap, that could also mean compensation for any consequences of that degraded capacity.
I remember when people thought BitTorrent would revolutionize file hosting. Now people think it's stealing bandwidth.
Don't know why ms didn't play it safe and put internet resharing as an opt in...
Oh and I don't remeber but I think you can't set a wired connection as metered.