Weirdly, middle-click (at least for me) does open the link in a new tab. If I were to venture a guess, it would be that that behavior was unintentional. If it wasn't, I'm also curious about the logic behind it.
Just in case someone from Optimizely looks at this: I was curious so I examined the JS source. It appears the "munchkin.js" helper is the most likely candidate. The code is obfuscated but I see a click event listener that is doing logic related to detecting the control key and simulating a mouse event. It's probably broken on Macs.
(In general, intercepting browser events and generating your own interacts poorly with obscure behaviors like ctl-click as well with popup blockers.)
