S3 E01 Freezing the courtroom

In the episode "The honeymoon's over", when Piper freezes the courtroom, why do all the other demons except Cole not get frozen? Edit: Just finished the episode and saw that Cole was pretending to be frozen. So in effect she only froze Darryl and Leo, and none of the demons lol

Charmed has serious continuity issues but how do they try to explain this one? Piper mentioned that demons getting "immunity", but how does that happen?