There is a reason for that, though.
If you can shoot while sprinting, there is no real need to walk/jog anymore. It's a balancing-issue. Especially in Battlefield, because of it's wide areas.
If all you know is "modern shooters" like Battlefield and MW2, perhaps your logic would make sense, but Brink isn't like Battlefield, with vast open area with vehicle support and gun mechanincs that favor realism over un-realism/fun and it isn't like MW2, where camping is promoted throught the implementation of kill-streaks and poor weapon balance (OSK in the foot, lol). In ET, sprinting was balanced with a sprint meter, so you couldn't do it all the time. Also, sprinting tends to lessen weapon accuracy or just makes it harder to keep your aim on targets in general, still making walking a viable choice. Brink also has footsteps, so sneaking up on players or trying to be stealthy makes walking viable as well. Other than Battlefield (which has such a strong, useless penalty against sprinting and shooting) I never usually walk in games anyway. It's either full sprint or crouching.
My point is, Brink is not Battlefield (or similar games) it seem to have more of an "oldschool PC shooter merged with modern shooter mechanincs," feel and the "moderen mechanincs" seem to only be included to draw the attention of newer games (and console players, who have not expereinced the perfection of SD-style team objective shooters.) Sprinting while shooting has been done before, numerous times, with complete balance intact. It is also a feature many SD fans are requesting to be included.