The UBCS (Umbrella Biohazard Countermeasure Service - can't remember when it's explained off hand but it's certainly not USBC), as stated, is there to counteract biohazards. So you'd think they'd be a bit more appropriate for the job of a viral outbreak, they'll be briefed on the mission and who or what they'll find (although it's a possibility they may have been misled or misinformed purposely, what with Umbrella's real intentions with them being in the city and all).
The military, as Brad so correctly says. Are "not trained for this kind of situation", so they didn't know what to expect. Sure, you'd probably figure out shooting them in the head is more effective in real life and not blocking your only retreat path like a maze with cars against shambling corpses who back you into corners, but this is Resident Evil, ineptitude is of a colossal measure here. That being said, if they did join the main fray, I don't think it would've helped much.
