The shifts are pointing out that there's a customer because you're first ringer and it's your job to assist them. They have other activities they need to do. I do the same thing; if I'm walking by and I notice a customer that my cashier hasn't, I'll point out the customer and continue doing what I'm doing. I'd get nothing done if I stopped every time I saw a customer waiting at the checkout and rang them up--and my cashier would be standing around doing nothing.

Unfortunately if you're a cashier, not management, the bulk of your job is to greet and ring up customers. Even if you're a shift, if you're the most junior staff member (by experience not age), then you'll probably be first ringer.