There's a simple way to explain it to them...
"If I build the lock with a way to open it other than the key I provide to the customer whom buys the lock, then *anyone* with that spare key can open that lock. So if I make a lock & give it more than one set of keys, a second set that could bloom to as many sets as there are users on the internet, would you REALLY like me to install that lock on your daughter's Chastity Belt? Because she'll be just as fucked as everyone else whom uses that brand of lock, knowing every bastard on the planet could potentially have the spare key."
Watch how fast the bastard backpeddles on wanting that backdoor installed & try not to laugh too hard at the smoke blowing, hand waving, "That'll never happen! We're the Government & we're Secure!" style bullshit starts to flow out their mouths.
Idiots, the majority of them. At least we've got one or two with a brain they've got the balls to use in our defense. The rest of them need to go up against the wall when the Revolution comes.