If that sign next to the tee said it was a par 3, would everyone's strategy change? Certainly 280 isn't too far to be called a par 3 in a US Open.
I think part of the reason for the somewhat overconservative play is the hole being labeled a par 4. When a hole is listed as a par 4, the default "smart play" becomes to lay up while the "aggressive play" is to go after the green. I see this at my home course. Our 300 yard home hole was originally designed as a 251 yard par 3 back in 1923. Today, the older members sometimes lament these young guys who bomb it at the green and the way that "equipment makes it play differently than it was designed." Little do they know, the green was designed to be reached from the tee.
The other part is the angle. It swings so hard from right to left that it's obviously gonna be hard to stop a driver or 3w on the green. I think in turn that makes going for the green seem like a low percentage play, but in reality, you're probably more likely to chip your second to within 10 feet than you are to throw an 80 yard wedge to the same distance to make birdie.