It isn't legislation from the bench merely because the Court strikes down a law. Rather, when the law violates a plainly stated constitutional right like free speech, to not strike it down is legislating from the bench in the worst way. That is, the Court is usurping the legislature's power to amend the Constitution. Legislating from the bench generally takes the form of striking down legislation because of ambiguity or infringing on some invented Constitutional right.
This decision nullified the most clearly unconstitutional part of the McCain/Feingold bill. It stopped short of striking down the entire bill, so I don't think it was activist at all.
If they fine-tune it, tone it down, adjust it, etc. that's a problem.