Bolograph - First, please refer to my earlier post regarding how managers and supervisors recommend spot checks.
Second - 'knowingly allowing' is sort of the same as 'reason to believe'. Basically if there is any doubt, the cashier should always ask for ID. Like I said, the law states that any shop that sells alcohol must take reasonable precautions to prevent the sale of alcohol (see my earlier post for this as well).
Third - Refer to my earlier post about a) spot checks and b) the right the check anyone because it falls under 'taking reasonable precautions to prevent to sale of alcohol to those under age'.
In law, "reason to believe" is not just "you can construct a ludicrous hypothesis involving the Loch Ness Monster", it has to be a judgement that a reasonable person in possession of the same facts might make. So when the police are only permitted to search people on the basis of having "reason to believe" that an offence is being committed, "he might be a shape-shifting alien who is actually Lord Lucan" while, of course, in principle true doesn't pass the test.
You are being rather ridiculous. Do you think the cashier wants to ask you for ID for shits and giggles? If they have any doubt as to whether the alcohol is to be given to somebody underage, they need to ask for any relevant ID. Their job depends on it. That's where 'knowingly' comes in. They must be damn sure the person they let by that alcohol is over 18. The 'knowingly' is more if the person uses a very convincing fake ID. If they asked for ID and followed the policy, they will be protected. If they didn't bother, then they're open to prosecution.
OP was not a 'victim'. They were just part of necessary protocol which has been in effect for quite some time. OP might have said or done something that indicated it - even talking to the daughter about being back from Uni or talking about a party they're going to throw might make the cashier suspect the alcohol will be shared with them. Beside her daughter has been to Uni she must be used to needing ID.
Or the case of the handicapped shopper told her child couldn't help load or carry bags that contained alcohol: what "reason to believe" was generated?
As for this, I never said this was acceptable. The staff member who did this was not complying with store policy. Once you have agreed to the sale of alcohol, how they take it to their car. I did not say even once that 'reasonable belief' applied here. Even still, it is in no way applicable to OP's situation. This is something else entirely.
Cashiers are not mind readers - they do not have magical powers that tell them how old people are and exactly what a customers intent is with the alcohol, and with so much at stake, they err on the side of caution. You would too. So if that means you and everyone you are with get asked for ID, then that's what needs to happen. If you had the threat of being fired, put in prison and fined, you would too.