I suspect this is Kennedy's fault. He probably was unwilling to join a broader ruling, so Roberts could only get a majority by limiting the decision to the issue of bias at the hearing.
If you read through the oral arguments, it was obvious that this is exactly what Kennedy was going for. I brought it up on FR at the time after I'd read through it. It was also pretty obvious that for the 'conservatives' on the court, that they were pretty much begging for a way to get past prior precedent on the 'public accomodation' angle. They are boxed into a corner on that and know it.
So, the best example of that is a straight person comes into a bakery, and asks to order a cake for the wedding of some gay friends. The baker refuses. in that case, the baker is not discriminating against the customer on the basis of sexual orientation, but rather refusing to make a certain product regardless of who orders it. So, they could make the same refusal if a gay couple came in to order that cake.
Conversely, if a gay couple came in to order a cake for the wedding of a straight friend, the baker could not refuse because that would be discriminating against the customer.