At it's worse the kid is just being a retard and is clearly trying to provoke a response, the problem is there are other retards who still after all this time haven't caught on to that behaviour so there are times when it is still funny.
What I don't like though is when it's obvious harassment and following people around, you're going to get shot doing that sort of thing. You also definitely don't deliberately start shit with people if they're clearly bothered by it.
This incident is just a boomer being a boomer, what he should have done is simply not given in and asked for his receipt. Though I suspect given how these pranks go the supermarket is clearly doing zero security checks and doesn't even realise he's probably not an employee, or if he is he's going to get fired hard.
Alternatively he could be trying to get fired LOL.
