Unlocked Tmobile S5, hot spot app works with AT&T?

[AC Question]

I have at&t unlimited, so I can't tether or hot spot...until now. Just got my unlocked Tmobile S5, and the Tmobile hotspot fired right up and I'm typing on my old s4 now with no SIM. So is this an awesome bene, or should I consider myself screwed when my AT&T bill comes in. Right now I'm tethering through pdanet very sporadically through usb with no repurcussions.

Does anyone know how this happened? Worked?