If they don't do it then you didn't actually them do it. You merely gave permission. Letting somebody do something requires you to willingly not interfere while they actually do it.
Since when? What part of letting somebody act requires that they do it? It's an act on my part solely, "let" is not code for "make".
Who said anything about making them do it? I certainly didn't. If the deed is never done you didn't actually let anyone do anything. You would have let them, if they tried. But would have isn't the same as actually allowing it to happen. You couldn't possibly have allowed something that never happened.
I can't rightfully claim that I allowed my sister to have an abortion if she never actually had one. I can say that I would allow her to have to have abortion, but not that I did allow her.
To truly allow something to happen requires both the act to happen as well as you choosing not to try and stop it. Otherwise you're just stuck at would have allowed it to happen.
Why are you in charge of your sisters abortions...? ---
Que sera, sera. Whatever happens, happens. ...and he was never heard from again.