I suspect he made the mistake of assuming they would want something he was not prepared to make. As the saying goes, assume can make a behind out of you and me. But a faulty assumption seems very little to base this case on. 

We all know he didn't bake the cake because of his religious beliefs. The question is, does he have a right under the constitution to refuse to bake a custom cake because of his religious beliefs? I think he does have the individual right to do so, but we'll have to see what the SC says.