Ultra Watch

So,

I have had an iwatch 4 for many years and have decided to purchase an Ultra 2. I have never needed an additional line my watch has been paired to my phone, that has been all I have ever needed. I went into T-Mobile today to try and purchase an Ultra 2 and they would not sell it to me telling me I had to purchase another line. I told them I have had a iwatch forever that has been paired to my phone and that is all I have ever needed. They said no, we cannot sell it to you. My question is why? I can easily go to Amazon purchase one and pair it to my phone, I went to my carrier so I could get the exact model I wanted. Is T-Mobile just trying to get more out of me to pay for another line??? I am really disappointed right now.