Both companies are making a huge push for in home internet with FIOS and whatever AT&Ts is called. This is exactly why your data is going up on your phone. They dont want to cannibalize their in home market with people having unlimited tethering plans. Heck, most people that tether either jailbroke their phone or found another hack to do so.
Its all about the money for in-home internet, not about network congestion. Dont believe a word they say. Everyone sees what Sprint is doing with its 4G wimax network... People will be using it has their home internet service.
ATT's is called U-Verse. I don't think that is their plan. Capping bandwidth to keep home ISP market open. I say that because not all areas have those services and it also leaves out many, many apartment buildings because they would have to wire fiber optic to each apartment.
Not to mention home ISPs have caps on their usage also. The most generous cap is from Comcast, who gives you 250 gigs a month cap. Others vary from 15-25 gigs.