One reason is that the decision can do them personally and other Republicans generally, no harm. The other reason is that the members of the Westboro church are rabidly anti-gay, which is a matter of ideological principle for Republicans.
From the decision: "Speech deals with public concern"that is, a subject of general interest and of value and concern to the public."
How union members are to go about their work, under what conditions and how they are to be compensated for that work is most certainly of value and concern to the public. The rights to decent working conditions and compensation affect all people who work for a living, and not just union members, so it of general interest and concern to the public.
Inevitably the Republicans will get a law passed somewhere that abridges the free speech bargaining rights of unions. And, inevitably, it will reach the Supreme Court. Then, the Republicans on the court will demonstrate their utter insensitivity to blatant hypocrisy by ruling for the Republican law and against the union.
Republican hypocrisy is like what has been said about the poor, they and it will always be with us. In Mike Huckabee's most recent hypocrisy he castigated Natalie Portman's single motherhood while he made no mention of Bristol Palin, who did exactly the same thing.
Again, like the Republicans on the court, the reason for his hypocrisy is quite simple.
Mike Huckabee is a Republican.
Bristol Palin is a Republican.
Natalie Portman is a Democrat.