But then, it seems that Barriss was acting against Palps's agendas, since she saw -- rightly -- that the Jedi had been drawn off their path by the Dark Side, manipulated into waging an unnecessary war. So it's odd that Palps and Tarkin were so eager to find a scapegoat.
Maybe he foresaw Anakin bringing her in in time? I mean foreseeing is supposed to be one of his more notable powers, no? On the other hand having Ahsoka convicted or executed would certainly help push Anakin further towards the dark side. Even as it stands with Barris captured the reputation of the Jedi has been further tainted, making the purge all the more justifiable. So basically he had no reason to interfere as he wins either way.
Apropos of nothing, I like how they intimated that Ahsoka knows about Anakin and Padme. I wondered when they were going to get around to that.