T-Mobile and the iPad

I am not sure about the other mobile telephony providers, but this morning I went to T-Mobile with the intention of buying an iPad mini. They had only the dark colour in stock, and on 32GB capacity—I was wanting a 16GB. The transaction was almost finished, when I was asked for a telephone number. After stating that I just wanted the iPad, no connection, I was told they could only sell it, if it was with a mobile data plan. Business was lost.

Why? If I am buying an iPad, with a cellular radio set for T-Mobile frequencies, why to enforce activation right on the spot? It is absurd. The Apple Store¹, on the other hand, had plenty in all colours, all capacities and no other attachments.