I've been using T-mobile for about 5 years as well and have been tethering for about 3 years with no issue. I just have a regular data plan. No hotspot option or anything.
Interesting, din't know it was uncommon. Could they be targeting certain areas, maybe the ones with high data usage? It happened to me when I moved to NYC and found myself stuck in an apt with no internet for the first few days.
I had the same thing happen when I moved, after I racked up around 800MB of data on my tethering (easier than you think). I didn't get a text, but I'd get redirected to an "upgrade now!" page. I spoofed my user agent on my laptop for the next couple days (because, shit, I'm paying for it, I'm going to use it), and had to do the same when I tethered for about a month or it'd redirect me. Since then, nothing.
I've noticed that it redirects when I use a largish amount of data (100mb?) with a desktop useragent. It redirects me to the "upgrade now" page for the rest of the month.
I wonder why this isn't happening to everyone?