Can I bag my items as a shop with 'scan and go'?

I've been using scan and go for a couple of years. I've always bagged my items as I shop so all I have to do is scan my phone at the sco and be on my way. In the 2 years ive done this I've never had a problem except today. I scanned my phone to pay then the sco host came to me and said I wasn't supposed to bag as I shop then she began to paw through ALL of my groceries, while checking my receipt. Has that always been a rule? Why did they wait 2 years to tell me this?