I'm not sure that they've ever proven that Maxi stole the missing pills. I know they know that she 'bought' some.
But I guess if they bust Maxi for stealing and tampering with charts, etc. They would have to bust Liz and Eppy too for breaking hopital policy, not to mention violating a patient's privacy by looking in her file and threating to use the finds against her.
Well, I guess that was more Liz then Eppy though it was Eppy's suggestion.