It's purely a safety rule
If you can't see down your sights, you're blind firing. Simple as that.
I've seen people look through cracks while their gun is above cover and they can see where their BBs are going. Problem is they have no fucking clue they're aiming at your face from 30ft away until they shoot you in the face. Then rage ensues and someone gets banned or hurt.
The other issue is you have no idea what's going on on the other side of cover. More applicable to cover that's not a single wood pallet, but basically you don't know if someone is going to walk right in front of your barrel and take a shot to the neck point blank.
However, there IS a way around blind firing through those cracks. If you back up enough that you can see the peripheral around cover AND use your sights to aim, it's acceptable to shoot through the crack to the other side.