From what I have heard, that is SOP for the Secret Service guarding the presidential motorcade now. Yes, it is menacing, but I figure they are the experts are presidential security. Maybe things have changed since 9/11.
On Bush’s watch...never heard or saw it. Anyone?
"I was driving east on the freeway and noticed that there were no cars on the other side of the freeway. Then I remembered that Bush was coming to town for something or other, and judging by the cops on all the overpasses, I figured this was why no one was on the freeway. I pulled off near the Gateway Transit Center to watch, as many people already were. This Presidential motorcade was a lot longer and more incredible than the one I had previously witnessed downtown. First, about 5 motorcycle cops cruised down the freeway way ahead of the motorcade, then probably about 7-10 normal cops, followed by about 3 suvs with guys hanging out of the back with guns, followed by 3 Presidential limousines (with two flags on either side of the front of the limousine), followed by pretty much the mirror image of what had just preceded them. In addition, there were a few ambulances trailing the entire thing. It was really pretty crazy. I can't imagine living in Washington, DC if this kind of thing has to interrupt traffic all the time. Must be nice to be the President and never have to worry about traffic (well, and all the other perks.)" - link
"What you can't see in that photo is that there are dudes with machine guns in the back of the suv's. Was crossing the street here in NYC a few years ago when Cheney's motorcade went by and I was trying to jaywalk behind and secret service dudes in the open back of SUV aimed there guns at me, that'll teach me not to jaywalk." - link