But, if your teenager isn't with you at the time of purchase and you take it home and give it to them, unless you've said "This is for my 15 year old", that's all on you, the cashier doesn't stand to get the blame or the licence reviewed/revoked, fines imposed or sanctions applied to it for a proxy sale because they couldn't be seen as being able to reasonably suspect that's what you were going to do, and therefore they're not responsible for someone under age gaining access to alcohol.
If your teenager is with you, and you take it home and give it to them, then the cashier could be seen to have reasonably suspected that's what you were going to do and therefore are also responsible for someone under age getting access to alcohol.
The fact it's legal for you to do it is a moot point because it's still illegal for the cashier to sell it if a police/licencing officer or other official decides that the cashier should have reasonably known it was a proxy sale. And it's illegal for you to buy it if the person it's intended for is under 18.
Totally contradictory I agree, illegal to sell it, illegal to buy it but legal to give it in your own home over the age of 5. But the cashier is as bound by that as you are, they can't change it and a defence of "I was using common sense" "I was giving good customer service" isn't going to wash when the law, or the terms of the licence have been broken.
The people enforcing the licence terms and the law have no interest in common sense or customer service, they are there to ensure that anyone selling alcohol does so responsibly and within the terms of the law and the licence.
And the terms of any licence include a mandatory age verification policy, and you have to be seen as using it. Challenge 21 and 25 are considered best policy and least likely to result in under age sales. They're not the law, and they're not company policy, they're licence terms, the breaking of which can lead to an individual no longer allowed to serve alcohol (that's your job under threat then) fines for the person selling and the premises holder, review of the licence and restrictions imposed or ultimately loss of the licence - which are expensive to get and renew as well as the loss of income if you're no longer allowed to sell alcohol at all.
I'm not going to risk that so someone else can have a bottle of wine in all honesty, of course with that at stake the people selling it are going to err on the side of caution.
Alternatively we could always campaign for a change, in that the person buying the alcohol is solely responsible for where it ends up and anything that goes wrong and people can take responsibility for themselves. As someone who sells alcohol, I'd really much prefer that to the choice I have at the moment of being under threat of what is described above or being treated like shit by customers who have decided that I'm just being awkward for the sake of it and reactions ranging from being determined to prove me wrong to downright abusive and threatening.
No one died because they didn't get a bottle of Merlot to go with their dinner, it's really not the end of the world.