The Answer: Because the cake baker discrimated by NOT making a wedding cake for one specific group or class of people but made them for other group(s) or class(es) of people. His mistake. Now he makes none. Now he is following the law.

* Do For One, Do For All, or Do Not Do AT All. * 

Gays are not a special protected class. But they, as a class of folks, are protected from discrimination.