...is such a stupid concept.
If you think a law is constitutional and a judge says its unconstitutional, then you think that's "judicial activism". If he invalidates a law that you think is unconstitutional, it's obviously not judicial "activism" - he's just doing his damn job. In fact it MIGHT BE judicial activism if he consciously allows an unconstitutional law to stand.
If you think a judge generates new principles to force on society, he's similarly "legislating from the bench". However, if you thought those principles were already inherent in the Constitution, he's not "legislating from the bench", he's once again just doing his job.
So whenever anyone talks about judicial activism, they really are saying nothing whatsoever about the quality of the judge - they're simply drawing attention to the fact that they disagree with the judge on the Constitution, and they think the judge is wrong.
Why can't we just say "that judge is wrong"?
The same goes for originalism, by the way. I guess there might be some people out there who genuinely don't give a hoot about the Constitution, but most Americans who engage in intelligent conversation about this stuff want a constitutionally limited government and acknowledge the Constitution as the legitimate limiting document. "Originalism", therefore, is about as vaccuous a term as "judicial activism". Everyone is an "originalist". They just disagree on what that original perspective was. For some reason "judicial activism" is still actively used by either side (even though the term is completely plastic), while "originalism" has been pretty much monopolized by just one of the two sides that supports their view of the original intent of the Constitution. Two sides support originalism, in other words - only one regulalry uses the term or has it applied to them.