From what I've seen "unlimited" hosting providers will simply limit you in another dimension, which I think is just fine. For e.g I frequently see bandwidth sold in two tiers: you can have "unlimited" transfer at 100Mbit/s, or metered transfer at 1Gbit/s.
This approach let's the host plan capacity while giving users the illusion of unlimited bandwidth. Meanwhile customers that genuinely need the extra throughput can still get it at a premium.
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This is essentially what T-Mobile is doing, they're just not being transparent about it. You can have "unlimited" bandwidth at ~3.5Mbit/s (a 480p video stream), or you can pay a premium to saturate your link. They're just choosing to apply the throughput restriction to a specific class of traffic (HD streaming) because (A) it's a large fraction of their total traffic and (B) it's fairly easy to market to customers.
Of course there is no such thing as truly unlimited, but it can certainly be "unlimited" in one dimension. The issue here is that's not how T-Mobile is marketing it, they're marketing their network as "the fastest LTE network"[1] while also saying you have unlimited monthly bandwidth. The latter may be true, but it comes at the expense of throttling the throughput (for some fraction of subscribers); and once you throttle a subscriber you can no longer say you're providing them with access to "the fastest LTE network in America."
[1]: https://newsroom.t-mobile.com/news-and-blogs/lte-advanced.ht...