What T-mobile support told you is correct.
The technical answer is because T-Mobile USA works under 1700 MHz/2100 MHz for UMTS/HSDPA (3G).
You need BOTH bands to work on 3G.
Mini pro sk17a works under:
HSDPA 900 / 2100 Mhz
HSDPA 850 / 1900 / 2100 Mhz
As you can see, both versions lack the 1700 Mhz frequency band.
I'm still with T-Mobile, although I'm planning to change to AT&T next month...