Also as the QNH is a mandatory readback item, if you have it set wrong, don't read it back and we don't challenge it - then you get airborne and cause an airprox because of it, it's our fault.
Technically.
Of course if the QNH changes all we do if you're on GMC or TWR is a broadcast and presume you've heard it - if you then cause an airprox because it's wrong it's still our fault. (I guess the theory is checking it once in delivery means you definitely have it set right once and then if it changes it will only be by 1 millibar so that's unlikely to cause a big problem if you have it wrong?)