Why does it cost more for unlimited data when I have my own modem compared to using an Xfinity modem?

I was paying for a plan that provided unlimited internet data. I recently replaced my Xfinity modem with my own modem. When the salesperson at the Xfinity store took the modem back and told me he would change my account to remove the monthly billing for the modem, I was never told my unlimited data was going away as well. I just found out I was about to exceed my limit, so I went back to the store and was told I no longer had it since I wasn't using their modem, but I could add it for $30/month. I was only paying around $10-$11/month before, plus the cost of the modem. What happened? Why was I not told this was going to occur and why would it be so much more?

I have been a long time Comcast/Xfinity customer and I am very disappointed in this.

This seems like a scam. Can anyone explain why it would cost more?