Shopping (again)

I always thought special offers were supposed to be optional? Today I was out walking in the countryside maintaining some of my geocaches, and popped into a foreign Tescos (ie not my usual one) for a sandwich and a drink on the way home. That’s all I wanted, a sandwich and a drink. The woman behind the counter became quite insistent that I buy a bag of crisps as well to make it a meal deal that she confused me and I ended up walking out of the shop, having not bought anything. Why couldn’t she just sell me what I wanted?