iPhone They saw me tethering..

Hi,

I have an iPhone 5 and a sim card from T mobile USA. I have the 30$ unlimited web plan.
When I was tethering to my laptop I got a message from t mobile informing me that a tethering option can be purchased online..
How could they know? I installed MyWi from Cydia and used the free trial..
Is there any other way to give internet to my laptop ( via usb/bt AND wifi)?
I would like to tether:
-wifi gogo connection from iphone(cheaper) to my laptop via bt or usb
-my mobile data from T mobile to my laptop via bt/usb/wifi

Are they that smart? I thought it would be possible via Cydia somehow.

Come to think of it: I did tether to my iPad (also via wifi) and that worked without any problems. I only got the message when trying to tether to my laptop.

In above cases the inbuilt tether function from iOS was used...

So would it be fixed if I used a User Agent firefox add on and set my firefox to iPhone?
At least I would be able to surf through firefox on my laptop?

PDAnet could also be a solution if the above link is still accurate, although I did hear that it sometimes completely bricks iphones, requring a full restore

So what would be my safest bet? I'm paying 30$ and I just need web, I don't think they would be losing any money if I surf a little bit on my laptop as well.
Since it worked without a problem on my ipad, maybe I can do the user agent trick. I don't need it 24/7, maybe 2 hours a day with just some surfing(wifi at hotel not free)