[Q] Very confused... SIM Card T-Mobile/ AT&T topic

Well, this is very strange... I just unlocked my AT&T LG G2 after a complicated mess ( i don't want to be reminded of that horrible experience :laugh:) and just to give it a test i put in a DEACTIVATED T-Mobile SIM card, new and never used or activated. I saw that I was getting cell connection as if data could be used... ? So I turned data on out of curiosity; and I got it! So where is this mysterious 3G/4G coming from and who is paying for it? It can't be me because I haven't even signed up for T-Mobile yet! Anything to clear this haze of confusion would be much obliged! Could it possibly be that T-Mobile has confused my G2 as a tablet and offered me the free 200mb of data to use per month? Very confused as I can't make calls or texts but data is working...