I get the “suffer” duality — you allow something even if it pains or inconveniences you. But I don’t get that “let” means “don’t let”. Thanks for ‘splainin’, tho.
It must be like “sanction” — which can mean approve or disapprove.
The two different "let"s originated from two different words - the one meaning "let go" or "loose" from the Old English lætan, while the one meaning hinder or hold back is from the Old English letten. Must have confused the Anglo Saxons too.