It's not really recoil though if you want to pull out the dictionary.
There is a big piston in some air rifles, the piston moves forward and comes to an abrupt stop at the end of the cylinder. That translates to the rifle jumping forward in your hands vs. backward like in rifle using a propellant charge.
That sharp forward snap is what kills the optic as they're built to withstand an impulse of force traveling in the other direction.
Back on topic there was an actual M-16 style pump up air rifle at the local gunshow this weekend. It looked truly awful.
Airsoft, where nothing is hurt but feelings.