I had a similar incident very recently. I was JS out of LAS on DAL. The gate agent informed me that the Delta gates do not have internet access, therefore they could not verify me in CASS and I could only get a seat in the back. I did get a ride home (thanks BTW), but if the cabin was full I would not have been able to ride the actual JS because they have no means to verify me in CASS. I asked if it was a temporary problem and she said that's the way it has always been at LAS. I asked if the front ticket counter could access CASS and she told me no, it was the same out there.
So, what's the skinny? Was she blowing smoke up my skirt or is there really a CASS problem with DAL in LAS? If so, is anybody addressing it?