That page doesn't completely account for other legal factors. See the page on Wikipedia on
Posse Comitatus for more details, specifically, the assertion that the Posse Comitatus act that the article refers to does not apply to the Marines (Reading the text of the law, which was actually linked to in the blog, that appears to be a correct assertion). The page on Wikipedia mentions that a directive from the Department of Defense restricts the Marines from acting as police, but doesn't go into details as to what sort of exceptions might apply.
As for whether or not it's
right, practically speaking, nuclear1's explanation makes a lot of sense as far as keeping the peace goes. A car full of drunk marines are a lot more likely to be able to cause trouble than a car full of drunk civilians.
If Bush's master plan for world domination is to do us all a favor by locking up drunk drivers, well, I can't say that I have many complaints. At least for this stage, anyway.