I want to know why I am not allowed to self install a cable box if I already successfully installed the internet

After giving up on the online ordering process I went to the Xfinity store.  They said I had to have a technician come out (for $100) to install the box since their records didn't show a cable box at my address before.  However I sucessfully installed my modem/telephone unit successfully some months before so obviously the cable if fine.  Between the online ordering hassle and this nonsense I am starting to miss Optimum, the service at my previous address.  And they were horrible.