I think using a kitchen knife, bayonet or screw driver would be suitable items that should be allowed or can be used to open cans.

I like the other ideas that people have mentioned too about the food becoming wasted and loosing some of it, or becoming infected when using something other than a can opener. Would be a good counter balance to the mechanic.

Found a couple of buildings where I can't shoot through the windows. Mainly only found this in the main town just down the road from the Airfield.