Recycle! Or else...

Unlike in Kraków, in Sevenoaks, there's always something open when you need to buy odd groceries.
On Sunday nights, it's the Sainsbury's local at the petrol station by the railway (I told you I live at the dodgy end).
Last Sunday, I had a urgent need to buy emergency dark chocolate to make emergency chocolate caramel shortbread (I would have taken my own photo but... uh... we already ate it).

I selected suitable chocolate and took it to the cash desk.

Cashier: Hi
Me: Hi
Cashier: Do you have any petrol to pay for today?
Me: No
Cashier: Do you need any help with your packing today?
Me: uh... no it's fine, honestly.

Cashier: Mumble mumble mumble bags today?
Me: I'm sorry?
Cashier: Will you be reusing your bags today?
(I'm still not sure that this is exactly what she said)
Me: Ummm.... I suppose so... I usually use them for something.
Cashier (apologetically): We're supposed to ask everyone.

She didn't elaborate on why. And since she had just handed me the bag herself, clearly I wasn't going to be reusing it today. I was confused. What precisely did J Sainsbury want me to do with my plastic bag? And would I be held to account for my irresponsible bag use every time I approached the checkout? Should I start to think up interesting things to do with my shopping bags so that I have something to tell the cashier every time?

Life is complicated here.