I was under the impression that the Bifröst Bridge is just a bridge (wormhole) between the 9 worlds, and yet in the movie it can be used as a weapon.
The bridge output an enormous amount of energy, and when turned to full power, that energy was apparently destructive.
It's probably something akin to how if you stand five feet from a fire, you are warmed, but if you stand five inches from the same fire, you are burned: the intensity of the energy transfer was increased by orders of magnitude, turning the 'bridge' into a 'Death Star'.
It beats the hell out of a kawoosh.
edit: Additional Information: There's really no scientific reason for it, other than the increase in the energy transfer, because the Einstein-Rosen Bridge is a purely theoretical concept. As such, its properties are only postulated, and scriptwriters can and will twist them until they serve the purpose of the story. Hence, the wormhole becomes a massive magnetic storm, with multicolored lightning and the potential for genocide.
The real question is why did the Asgard build their wormhole junction with the capability to output enough power to destroy a realm? And why would the Frost Giants be stupid enough to wage war against an enemy who could eradicate them in the space of an hour, from beyond their own effective reach?
The comparison to an automobile or rocket is a good answer. As far as why the frost giants would be that "stupid", I think the question should actually be "Assuming the Jotun are aware of the destructive power of the Bifrost Bridge, why would they wage war against a people who control that much destructive power?" The Asgardians are meant to serve as protectors of the nine realms, which means ALL nine realms, even Jotunheim. We can assume that the Jotun are aware of this, and know that Odin would never use the Bifrost in that way. This is just the kind of attitude and behavior that he was attempting to discourage when he sent Thor to Midgard (Earth) to learn the value of thinking before you act and behaving in a manner befitting the protector of the Nine Realms.
"if you stand five feet from a fire, you are warmed, but if you stand five inches from the same fire, you are burned"
Actually, I tend to think it has more to do with the gravitational forces from Bifrost. It is, in effect a black hole (as we see when Thor and Loki are hanging off the remainder of the bridge) so if focused properly, say into a small enough point, the gravitational pull needed to warp space would be devastating.