Well I guess we can just throw out the instructions for EVERYTHING right? I'm off to lick a light socket and take 42 Extra Strength Tylenols.
The instructions say "don't run it in the rain." These things are costly electronic devices, and are by no means waterproof. If you want to risk frying your $1000 generator in the rain, that's your prerogative. Maybe you've got $1000 to burn. Many of us don't. Don't deride the people that choose to protect their investment.