Because they love their carrier for actually making moves to satisfy their customers.
And it isn't "throttled" unlimited data. It gets deprioritized, which is very different. You only will see slower speeds in very congested areas. At that point, data service would suck whether you're deprioritized or not. Pretty much, if you aren't in the middle of a huge city the majority of the time, you are not likely to see degraded service ever on the unlimited plan.
Also it's 14GB of tethered data which, considering unlimited YouTube, Netflix, and music streaming, would be hard to pass for many unless you're using your phone as your home internet service... Which is frowned upon by all carriers. It causes congestion. Thus, the "soft caps" on unlimited plans and limited tethering. AT&T doesn't let you tether at all on their unlimited plans with directv I don't think. Unless I read wrong when I was researching it for a friend with no ISPs where he is besides garbage satellite.
Are you sure that service sucks in your area? If you go to T-Mobile's site, a prepaid SIM kit is only $3.99 if you use the code "SIMDEAL" at checkout. Once you get this SIM card, you can stick it in any unlocked phone and use it to see the cell service you would get just like normal (you get signal bars and what cell technology you are getting like EDGE, LTE, etc just like normal) for unlimited speedtests on the T-Mobile network so you can test before you fully commit. There is no need to buy a plan for it or anything, just put the card right from the packaging into your phone when you receive it. No setup. I used this to verify that T-Mobile has no coverage where I am lol.
T-Mobile has come a LONG way in the past 3 years. Not yet as good as AT&T and Verizon but still better than Sprint. Do not doubt this carrier though. They are improving at incredible speed in order to catch up with the big two - and they're already rivaling in many areas.
Stop with the blanket statements. Every area is completely different in terms of cell coverage. Where my cousin is? T-Mobile rocks with cell service while I, on Verizon, had weak, unusable 1x and 3G the whole time. A friend of mine, AT&T is fantastic at their house, Verizon, again, terrible. Can barely grab a bar of LTE that always drops to unusable 3G. Where I am though, AT&T and Verizon are equal, Sprint is spotty 3G, T-Mobile is absolutely No Service.