Why wasn't Bairstow penalised for fooling Smith with fake fielding?

England wicketkeeper Jonny Bairstow faked collecting the ball which forced Steve Smith to dive in the 30th over of Australia's first innings in fifth Ashes Test. The umpires didn't penalise Bairstow under fake fielding rules as his action was considered to be done "out of jest" than out of "an attempt to cause confusion and prevent a run being scored".