Some of those vouchers are £100 so I'm surprised to see a big tag on a bottle of Baileys (£15) and yet say a £100 Amazon voucher that's small and would be ease to conceal dies not have any security attached to it.
This just goes to show your ignorance of how things work.
Expensive items like alcohol are tagged because they are high value and easily stolen - the tag alerts us before you leave the store so we don't lose a high value item. With a voucher, until it goes through a checkout it is worth nothing more than any other piece of cardboard as it doesn't get activated until the payment has completed. Therefore, we do not need to tag it as pre-checkout it holds zero value so you're welcome to steal it but it will be worth nothing.
As someone who works on self scan occasionally, the process is that we only detag items after payment. Our general policy is that we wait until the end of your transaction to verify age restricted products (ours let's you scan and continue, it just flags your self scan to us as one that will need verification) and then take them to detag them as you are paying.
This is done for multiple reasons but mainly:
a) Payment - to ensure it has been paid for - whether you have intentionally or accidentally not paid (you'd be surprised how often people's cards will decline because they have tapped too many times and it needs chip and PIN) if a member of staff has to come to remove any tags after you have paid, we can ensure the transaction has actually completed and we aren't letting a lot of high value items leave the store unpaid for.
b) Efficiency - if I have to detag items after payment then I only have to come over once to take all of the tagged items to detag them enmass. Again you would be surprised how many people have multiple tagged items and expect you to just go back and forward to detag each item individually as they scan it. I will even tell people who scan a restricted item to carry on and then when they have finished shopping all I have to do is authorise their item on the screen and then, while they are paying, I can take the items to detag and by the time I have returned I can see the transaction has completed.
C) Extra opportunity for verification- if I come over at the end of a transaction I can immediately see which items have tags and take them to be detagged (when it's your job to see tagged items you get good at it). If I am also verifying the items then I can see if everything has been scanned - if there are 4 bottles of gin on the screen to be verified but 5 waiting to be detagged then I know one hasn't been scanned.
D) Security - If I detag everything I can see on your self scan scale but you still set off the security alarm when you leave the self scan area then I can alert security so that they can stop you (they are the only ones who can) and check that you haven't hidden a load of bottles of gin at the bottom of your bags that hasn't been paid for. If we have a good reason to believe it is a mistake/defective tag not detagging (eg you have an empty trolley, no handbag, small/minimal pockets for concealing items) then we can use our headset to alert security that they don't need to stop them and they can wave the customer through with apologies.