There are a few things wrong with both sides' arguments here:
1) People don't like to be called criminals (regardless of whether or not their actions skirt such a line). Call a pirate a pirate, and they'll take exception to it. Call someone who meant no harm, but are considered a pirate a pirate and they'll take exception to it. The reaction is all the same.
2) The way T-Mobile intended this to work is that data usage on a mobile phone would be unlimited, but if it's tethered to another device, it would be limited to 7 GB. I personally haven't read their ToS, but others are saying that their ToS clearly states this. So if T-Mobile is clear about this, then there's no real deception on their part. However, if the advertisements remain vague about this, but the tethering caveat is well-hidden in fine print, then one can argue some level of deception.
3) There are those that know what they're trying to do:
intentionally circumventing the tethering data limitation with the
clear and concise goal of avoiding additional charges. If people that fit under this profile are turning around and complaining about #2 (if T-Mobile is transparent) under the argument that they were NOT transparent, then the deception is coming from said customer. Is it theft? Depends on your definition. Is it fraud? I think that would be a more accurate term*. Is it abuse? That would be even more accurate**. However, see #1 above about people not liking to be called criminals (or really any negative term that would damage their self-image for that matter) and that would easily explain the downvotes in this comment.
4) There is an old saying: "Give them an inch, and they'll take a mile." T-Mobile needed to plan for something like this--security, PR, etc. Again, see #1 about calling people criminals, regardless of whether or not they fit the profile, and why this is not a good idea from a PR perspective***.
=========================
*: An example of fraud would be purchasing a PlayStation 4 or an XBox One, and then after the warranty expires, purchase a new PS4 or XBone, packaging up the first console in the second console's box, and taking advantage of the second console's warranty to replace the first one. It's a method of deception that may be harmless to us consumers (especially since we see companies as filthy rich and can afford losses without flinching, and whether or that that's actually true doesn't matter at all to us), but in reality, is a criminal offense. In this case, we would be criminals whether we want to hear it or not.
**: An example of abuse of good-will would be going to an all-you-can-eat buffet (as someone previously pointed out), and sneaking the food out of the restaurant to fill up their refrigerator, freezer, etc. in lieu of grocery shopping. Yes, they advertised "all-you-can-eat," but I don't think they can stay in business if everyone decided to sneak buffet food out and have that feed a family of 5 for 2-3 weeks.