It is nothing but slavery to be forced to do someone else's bidding.
It damn sure isn't freedom.
Yeah, right. I suppose it's slavery then, if a railroad is forced, under penalty of criminal prosecution, to operate with certain safety features in place, or to be required to offer its services to all comers (i.e., to be a "common carrier").
Requiring that a business be operated according to certain rules and regulations, and subjecting business owners to fines and penalties if they violate those rules and regulations, is not slavery.
The bakers were never forced into baking cakes; they had a simple choice: follow the laws that apply to the commercial activity of baking cakes, or don't get into the business of baking cakes for profit.
The penalty, of course, seems outlandish and quite possibly unconstitutionally harsh given the minimal harm suffered by the folks who didn't get a cake baked. Attacking that seems to me to be the better route.