Heck not too long ago the muslim cab drivers wouldn't even allow people with dogs in their cabs. Because dogs are unclean to them.
So the question stands, why can one group say it is their religious belief to not do something and that's okay, and another group says it is against their religious belief to do something and they are stripped of their business, fined, etc.? Both are invoking their freedom of Religion, and one is invoking their freedom of speech (expression), which is another part of the First Amendment. While some Muslim cab drivers may ply their trade creatively, there is no doubt that creating a wedding cake requires creativity and talent, therefore, that cake is an expression. Forcing someone to express something that goes against their deeply held (religious) beliefs is a violation of their Right.