The country seems to have gone a bit mad.
Indeed it does, people want the shops and the people selling it to be accountable when something goes wrong, but also don't want to have the issues around proxy sales affect them at all and start talking about 'common sense' and sales assistants being power mad and stupid when refusing a sale on the suspicion of a proxy sale when buying it with a teen in tow. Or the same when someone is challenged under challenge 25 because "The law is 18!!".
But want those policies to be effective so that a 15 year old can't get access to the alcohol to get in this state in the first place, even though they are completely unwilling to accept that it has to apply to everyone, including themselves, to be effective and stop proxy sales happening.
This is a pretty straightforward one as the person who actually bought the alcohol was under 18, so shouldn't have been able to buy it in the first place for it to become a 'proxy' sale for being bought for a 15 year old.
So if you want to be able to buy alcohol unhindered by being ID'd if you look under 25 to the person selling it (and taking the responsibility), because 18 is the law, or with your teen because you're over 18 and a responsible parent and should be allowed to buy alcohol if you want, then you need to accept that mistakes will happen and someone under 18 will occasionally be sold to or proxy sales will happen.
Or if you want to ensure that the risk of an underage sale or proxy sale is as low as possible, then one day you might be asked to provide ID or refused a sale if you have an underage (or without ID) teen with you.