Tethering is built into Android from Version 2.3 (or even 2.3), no marketplace apps needed.
Anyway, I don't understand what Google is supposed to do if carriers choose to disable it. Maybe customers should be looking into other carriers if they don't like the way their carriers conduct business.
It was there in "stock" 2.2 as well, on the Nexus One --- but disabled in almost all carrier builds.
And Google does have some leverage, if they choose to use it, in the Market compatibility requirements --- there are minimum requirements for both hardware and software if you want to get the Market, or Google's other proprietary apps. (Which, say, the Nook Color flunks --- no camera.) It might be a bridge too far for some of the carriers, but from the outside, it's not obvious that they're trying.
(And looking for other carriers is not a hugely practical option in an environment where there are basically four, and the one with the most generally consumer-friendly policies, T-mobile, is up for buyout by the one with the least, AT&T, which is the only one to ban sideloading altogether.)
Not sure if access to the market is much of a leverage. I think most players in the mobile game seek to establish their own markets anyway. The phone makers certainly do.
There are no carriers in the US that don't play these games. Sprint is a little better about not limiting the stuff they label as Unlimited, but they still charge for tethering, have absurd overage fees if you're not on an unlimited plan, and still have Sprint/OEM specific Android builds.
I haven't had any charges on T-Mobile as of yet. That being said I've only used the hotspot and tethering a handful of times so I may not have hit some unforeseen limit.
Tethering on T-Mobile is free. I don't have cable/DSL at home. Rather, I tether through my T-Mobile cell phone exclusively. Word on the street (T-Mobile forums) is that there is a 5 GB soft cap on data. Once you hit it, you are dropped to Edge speeds.
Last month I went over 5 GB for the first time (5.5 GB) but speeds were not dropped.
Anyway, I don't understand what Google is supposed to do if carriers choose to disable it. Maybe customers should be looking into other carriers if they don't like the way their carriers conduct business.