The 36-year-old is contracted with the Stars for next BBL season and is believed to be on a deal worth around $200,000 that includes TV commentary and promotion of the BBL.
Pietersen, who also limped off with a sore hamstring while fielding, was Johnson's third victim after the left-arm paceman's two wickets in the first over of the match immediately put the Stars on the back foot.
On the player's mic, he noted the WACA pitch was unusually slow before mistiming a wristy leg-side shot and was caught at mid-wicket.
Stars board member and chairman of selectors Damien Fleming defended Pietersen's contribution at the franchise in TV commentary.
"He does a lot of marketing, a lot of promotion. Up until now the Melbourne Stars are very happy with the way he has contributed," Fleming said.
The Scorchers will host the final on Saturday night against the winner of Wednesday night's clash between the Brisbane Heat and Sydney Sixers at the Gabba.
- AAP