If the Muslim butcher usually carries pork, then yes, I would expect that he should sell it.  However if that same butcher doesn't serve pork, then, no.  In the cake case, they were asking for something he already makes, a cake, period.  The only thing that might be different might be the topper, and they could have just as easily provided it, or not had one at all.  So your analogy falls flat on its face.