Tethering is stealing
Violation of ToS
Tethering is fundamentally not a service. Neither product, nor service are provided outside of the existing plan. It's merely a (financed) license to use the data in a way that does not violate a ToS.
A breach of contract or ToS != theft. You subscribe to a data plan, which is presented as "unlimited". Regardless of the terms and conditions that may contradict this description, ultimately, you are accessing data. By restricting the means in which your consumer accesses said data, you are artificially creating an additional product/service. In of itself, tethering carries no intrinsic value, with exception to the arbitrary value you have assigned to it through its exclusion from the ToS.
Consider the following pertinent example:
Since we're talking about data, let's consider a topic with which many of us are familiar. When we purchase music, whether that takes the form of a physical medium (e.g. CD/DVD) or a virtual medium (e.g. digital tracks), we are subscribing to a license to access the music data associated with a particular album or track. We do not in fact own the track; merely, we own the rights to access the data as we see fit. Can you imagine if record companies proceeded to restrict the types of devices in which we could access this data? Say, if you purchased music from Sony BMG, you could only listen to said music on a Sony device, or only on a certain number of devices, or only a certain number of times? Let's say you would have to pay an additional fee to access your music data on an additional device or for additional replays. Tell me, if such restrictions were in place, how then would it be stealing? Because you're denying the potential for an additional avenue from which to profit? Let me reiterate what I stated before. By restricting the means in which your consumer accesses data, you are artificially creating an additional product/service. Remember, you're not purchasing the data, you're purchasing a license. The license to access the music carries no intrinsic value. Instead, it carries an arbitrary value that enables you to access the data in a way that does not violate their terms of service.
Truth be told, record companies have attempted this approach in the past. Does DRM ring a bell? Sure, it worked for a while, since many consumers felt they had no alternative to accessing music, short of piracy. Then something miraculous happened. The free market prevailed. Stores such as Amazon began offering DRM-free tracks, digital companies started offering various types of monthly/annual music subscriptions, and some artists have even opted to remove the middleman and offer their music for a more reasonable price or as donationware. The point is, the market created alternatives. Choice is a powerful concept. The interests of consumers are ephemeral. In order to stay relevant, companies must seek to appeal to those interests if they hope to survive. DRM has taken a back seat and is unlikely to ever be as restrictive or as prevalent as it once had been.
Digital products will always remain a gray area in the legal space, especially since when we're considering data, it encompasses vague laws and acts that frankly are no longer relevant or explicit. The ways in which we access our data is continually evolving, and companies are taking advantage of unclear legal language to concede as little into the consumer rights domain as possible. By creating an artificial product, they are able to negotiate a means of retaining the upper hand on their service. Essentially, they are biting the hands that feed them. If you want to believe that tethering is stealing, then so is playing the radio in public, or copying a CD for archival purposes. It's ultimately relative and a matter of perspective. From a legal standpoint, it's simply unclear. When you sell a service that guarantees access to data, why should it matter the means with which I use to gain that access? If you're concerned about excessive consumption, then define a limit, and don't call it "unlimited". In no part of the definition for the aforementioned word does it imply that restrictions are inclusive. Not only does that pertain to amount, that also pertains to means of access. A violation of contract is a civil matter, and in this case does not necessitate stealing, especially when the good/service in question is one that is artificially created by exclusion from what would otherwise be an inclusive service.
Carriers may be headed in this direction, to push as far into the consumer rights domain as possible, and this is exactly why we as consumers need to end our subscription to these contracts. The subsidized price of a phone may be attractive, but is shady and contradictory business practice a fair price to swallow? As with the case of music distribution, the market will ultimately decide the direction consumers will follow. Only time will tell what avenues will open.