The Smart was designed to be safely driven on german highways. You know, the kind where there are sections with no speed limits! So I don't see how US highways that are limited would pose any more problems.
You missed the part where since Mercedes couldn't come up with a small engine that would pass the US emission standards, they stuck the Mitsubishi 3B21 engine in it when they created the second generation car. That is the only engine available to US not-so-Smart buyers since 2008. We don't get the turbo or diesel versions, just the crap normally aspirated one.
The US NA version is further crippled and makes even less power than the Euro NA base engine. So, basically, you have to rev the holy crap out of the little three banger to get it to do anything on the highway. US ones also have a speed limiter that kicks in at 90mph, but it takes them forever to get there.
As for our limits... I have a helmet cam take from a recent highway drive where I was doing 80+ in a 70... and I was *still* getting passed. So, yeah, it's more a suggestion when there's no cops around.
Edit: And here's the video, just found where I'd put it.
[video=youtube;MLGPrWw-0ds]http://www.youtube.com/watch?v=MLGPrWw-0ds[/video]
Note that the SUVs are doing about 80 or so and even the little Prius is somewhere in the high seventies trying to do 80 as well. I'm not saying anything else about the not-so-Smart's highway manners other than they're audibly screaming their little hearts out to try to keep up with US traffic, and that this seems to result in elevated dead engine counts/shortened engine life per the Smart forums.
Edit 2: Also, the Smart three-banger makes one of the worst sounds in the world when it's being pressed hard. They should all be crushed for inflicting that sound on others.