Mmm, I don't think this is a question of legality. It's a question of morality, and what is deemed to be "right". It can be an entirely personal position, where the line to serve should be drawn. Maybe a predominately black bakery doesn't want to make a "kkk 4 eva" cake. Maybe that same bakery also doesn't want to make a wedding cake celebrating an interracial couple. Let's pretend both of these actions were legal - do we, collectively as a society, find them both morally right?
That's where stuff like "voting with your wallet", community engagement, and going to the press comes in.
Article seems to categorize the incident as discrimination, which I do not believe is accurate.. legally speaking at least, it is not discrimination.
The morality discussion is an interesting one, because there are various scenarios you can apply to the topic, however, regardless of which groups are involved, the law must be applied equally. If one group can deny a design because it is a political affiliation they do not agree with, then so can the people of other political opinions. The law does not play favorites when it comes to political opinion. The law says you cannot deny a person service for their politics, race, religion, ect
but again, to deny a specific cake design is not one in the same as refusing to do business with an individual. I'm sure if the ex-Muslim group wanted to put "happy birthday" on their cake, there would have been no issue