Sainsbury's Self Service Tills Encouraging Use of Plastic Bags?

I am growing increasingly frustrated with the operation of Sainsbury's self service tills which seems to be completely at odds with the store's policy to reduce plastic bag usage.

The tills give the option of using your own bag. Several times I have put down my bag, containing my purse, phone and other items in order to add a few purchases to the bag. The till refused to work, saying there was a problem with my bag and to ask staff for assistance. A member of staff then came over and pressed a few buttons and I was able to complete my purchases. At no time did a member of staff tell me the till didn't like me puttting a bag down that already contained other items. My husband pionted out the problem with other items in the bag so on my next visit I had an empty Sainsbury's reusable bag with one empty carrier inside. The till still refused to work.

This systems seems to promote the use of Sainsbury's branded plastic carrier bags which are prominently placed on the tills whilst the reusable bags are further away, out of reach.

I have written to Sainsbury's and asked why the operation of the self service tills is so much at odds with their policy to reduce plastic bag usage.  Specifically I have asked the store:

1. Why can't I add a few purchases from Sainsbury's to my handbag?
2. What if I do some shopping at other stores then call into Sainsbury's for some bread or milk. Why can't I add these items to my bag?
3. Why don't staff tell customers what is causing the problem so they are at least forewarned for their next visit?

I will update this post with the reply I receive from the store.