We can't just say that "Snakes on a Plane is good" (which it is, by the way -- really good) because we don't want to be overly fawning in our praise of a lowbrow film. We're taught to think we can't honestly enjoy a movie that doesn't qualify as high art. We're basically worried about being discredited within a cynical industry for displaying what remains of our youthful enthusiasm.
The "so bad" part, then, is meant as a caveat to the critic's boundless, childlike joy. It's where the scornful, grown-up industry-insider wants to acknowledge that, while the film is great and completely satisfying, it definitely isn't high art and thus must have some kind of deep flaw. It's reactionary fear, it's hubris and, unfortunately, it's the status quo. It's also totally counter-productive.
The claim that low art is bad art is something pop music critics dismissed -- for the better -- decades ago. (Many, however, make the equally sticky error of attributing vast cultural and artistic significance to trifling artists. At least their enthusiasm is in the right place.) As a result, perfectly honest and fruitful comparisons can be made between the Velvet Underground and Ashlee Simpson. Try doing that between 8 1/2 and X-Men III.
Why it persists in commercial film criticism has to do with the (incorrect) belief that there's a sharp line of demarcation between the art house and the blockbuster, a misperception that films are either art or entertainment. More pointedly, it's the belief that entertainment for its own sake cannot be a truly transcendent experience. It's a crass distinction, but the guilt it causes (among critics and among a certain class of moviegoer in general) is real.
There's no doubt where Snakes on a Plane falls on the spectrum. It is meant as pure entertainment and yet is, at times, a sly satire on its genre without ever being above it. Like all good disaster films, it's inclusive of race and social standing. It assembles all its victims and then kills them one by one. Like most horror films, it kills them based on its own twisted sense of moral balance and fair play.
Yes, these are all the wankerish things I think when watching a film, and by the time the first snake latched on to that first lady's breast, I had a working thesis for my review. By the time a snake got peed on, then retaliated with the ol' fangs-to-the-genitals trick, though, I was laughing so spasmodically that I couldn't keep my mental place.
Which forced me to do something I don't often do at a movie: I sat back and watched it. I forgot about my thesis and started wishing instead that I'd brought along a plush snake. I left laughing, exchanging lines with others and generally enjoying myself. How's that for a transcendent experience? It wasn't until later that I tried to decide what I'd say about the film. I came up with this: Snakes on a Plane is so good, it's good.