Well first of all, I don't think It's been confirmed that players can do that. And second, I wouldn't call them stupid for not including it because it may be unbalanced or just make the multiplayer out of control.
Currently Playing: Modern Warfare Reflex FC: 0363-1767-1144-SlyFox
I say that the players should be allowed to do this by simply performing a melee on the object they want to knock over. No special action assignments need be added to the existing controls. Plus, it would be a nice touch if HVS allowed this in both single and multiplayer.
Uncharted 2 Machinima Voice Actor. Currently playing Sully and Lazarevic. Add me on PSN if you want to get involved: Geo_Chronic.