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.
Republicans on the court weren't foresighted enough to see how that free speech decision would apply to the union members right to exercise their free speech. The wording of the decision affirms the Westboro church's right to express hatred for all other people who don't agree with them. The same wording also affirms the union members' right to bargain in how they are to live their lives.
From the decision: "Speech deals with public concern"that is, a subject of general interest and of value 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.
Because, unions vote Democratic.
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.