It's not 'regulated' by UK law, nothing to do with that but it's what the retailers have chosen to do, it's probably as much to do with making sure they get paid as anything.
OK, but you also said:
However as I said before you do have to be over 18 to buy them, not 16.
Which certainly sounds like it's a mandatory thing.
You also said (I can dig up the quote, if you like) that you could expect to be stopped by the police for the heinous act of carrying a stick.