They were creating a hazard and operating a vehicle while impaired. Both good reasons to arrest them. Whether they were drunk or stoned should be irrelevant.
I agree. I am glad they were arrested. I have read some comments here also about pot being harmless or no big deal, Well, I damn near got shot with birdshot as I went dove hunting with a man I had no idea smoked the crap. He started acting funny and shot a damn butterfly four feet from my head. The SOB was so stoned he claimed he thought the butterfly was a dove. I left him there.