Why is it automatic when its a simi/burst fire weapon?

Beats me. But every assault rifle and submachine gun on that game are automatic only. This was when UBI really started to go away from realistic gameplay, although they surprisingly brought back the ability to select rate of fire (for the guns in the game that have it in real life) in Rainbow Six: Lockdown. It's still in Vegas, but I don't know why they took it out of RB63 and Black Arrow.