How is this different than the Christian baker that would not make a cake for a LGBYLMNOPQRSTUVWXYZ wedding?
The baker was running his/her own business. If the baker worked for a bakery and the bakery requested he/she make a cake for a gay wedding and the baker he/she should be fired.
The baker owns the business.
The baker was not in the business of selling cakes for celebrating deviant relationships; had never done so in the past; and never indicated in any of her business dealings that she was interested in or willing to engage in that business.
In this case, the person works for another business that has sold these products for years, advertises them to the public, etc. So the employee had full knowledge that these products were a part of the normal business conducted by their employer at the time he/she was hired. When the employee refused to do the job for which they were hired, the company did not fire the employee, but gave them another assignment that did not compel them to violate their religious principles (an option not given to the baker).
If a gay couple went to a baker and said, "Give me that cake" bought it and left, the baker wouldn't have a problem with it.
The cashier (who, by the way, doesn't own or run Costco) took a job at a place that sells pork and alcohol and expects people not to come to his register with pork and alcohol.