You pay for your water based on how much you consume. If you give water to your neighbor, you are completely justified doing so because you're paying for it.
Theoretically, if it were unlimited water for just your household, then it wouldn't be fair to use that same water in all of your houses around town (because, of course, you're filthy rich and own plenty of houses). While you would be getting unlimited water for your main household, you would still need to purchase water for your other houses. The word unlimited is perfectly acceptable in that situation, since you receive unlimited water for your main house. If your water company gave you 100 gallons of free water to use at your other houses, that doesn't negate the fact that you get unlimited water in your main household. This can be directly compared to what T-Mobile is doing with their unlimited data.
T-Mobile is giving users 7GB of hotspot data to use on additional devices. This stands alone from your unlimited phone data, and is a feature for their plans. This is like the water company giving you 100 gallons of free water for additional houses.
T-Mobile is going after users that are using their phone data for all of their devices, which is not what you are paying for, nor what is advertised. T-Mobile is completely justified going after these people and their wording is perfectly acceptable.
On the other hand, I find it misleading that T-Mobile says that all of their plans have unlimited data. Their "throttled" data is slow enough that they shouldn't be allowed to call their plans unlimited. They should be forced to advertise their plans as "10GB of data with 64kbit throttled data thereafter." But that's a conversation for another day

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