I wouldn't say this was super open and shut. I'm not very surprised, honestly. She said she did it, but did she smoke way before she left? How much did she smoke? That's like saying "I drank some alcohol and then went to OG". Ok, but the alcohol was several hours before. Or maybe I drink all the time and it didn't affect me. Prove that my blood alcohol level was over legal limit... you can't.
I think the only reasonable way they could've gotten her is if they had video of her behaving erratically, or if they'd pulled her over and tested her. Even if they'd tested her a couple days later, it wouldn't have guaranteed she was too high to drive when she went to OG.
I just really think since she got off on this charge, she's going to be SUPER smug and think she's going to get off on the impending murder charges. She'll never give up the info now