Was the bag you were supposed to have offered free, or would the customer have to pay? If paying, then it's just a sales ploy. I'm afraid these are things you just have to get used to in the customer-facing service industry. Many of these 'initiatives' are dreamt up by staff who are not customer-facing in the hope that it will sound good and justify their existence.
I feel that all this was started by McDonalds (like so many other irritating phrases emanating from the USA, e.g 'anything else I can help you with?'). Although it is many years since I patronised one of their establishments, it used to annoy me intensely when buying a burger to be routinely asked, "Would you like fries with that?" My invariable response was to the effect that had I wished to do so I would have asked at the time of ordering. Yet the poor assistant, like yourself, would have been 'counselled', had the question not been asked.
To turn this on its head: until fairly recently, whenever I paid in Morrison's supermarket with a debit card I was always asked, "Do you want any cash back?", to which my answer was invariably 'No'. However, a few months ago, I found that actually I did need some money, but the question had not been asked. When asking for £20 I was told that my transactions had already been processed and would have to start again. On asking why, I was told that this facility was still available, but only on request, and staff had been instructed not to proactively offer this service. I am still waiting for a satisfactory explanation of this volte-face.