Because Clara had took the blame and accepted the impending punishment for it, trusting in her "get-out clause". She had in effect admitted guilt. There was no declaring her innocent after that. Despite the fact no crime had been committed. That's how the tattoo exchange worked.
It is still a pretty screwed up justice system. Me put the tatoo on Rigbsy despite knowing he was innocent on the grounds that she had to play by the rules of the street to keep the peace since the aliens all thought he was guilty. Now, Clara accepts the guilt of a fake murder so she must die even though she is truly innocent and all the aliens know it. And Me presumably could have removed the tatoo from Rigsby if his innocence could be proven, but she can't remove it from Clara who is clearly innocent? And wouldn't the Aliens be pissed at finding out that the murder never happened and they were all deceived as part of a trap? And why would Me promise Clara that she is safe when she knew that the tatoo could be transferred? Why not warn Clara not to take the tatoo because the death sentence will be final since she claims not to want anyone to get hurt?
I still think the episode had a lot of plot holes.