Personally, I think there is an element of time constraint and I can imagine situations where I would eat literally on the go. Usually I would prefer not to. Also, I think it depends a bit on the shop. Waitrose are very happy to make arrangements for parents to buy fruit, for example, that a small and hungry child can eat in the trolley while going round the store. I would not see eating food in a supermarket (that one has paid for) as being terribly bad manners, and I am sure that I have bought a few things over lunchtime and also a sandwich (paid for), which I have of necessity had to eat on the hoof in the supermarket.
I can see why clothes shops and some book shops would be reluctant about eating near their products, in case they got damaged. However some booksellers have cafes in them and positively encourage customers to have a coffee (and possibly a pastry) while browsing (Blackwells, Waterstones, Borders formerly). That has been a successful sales technique with me, as I have been able to sit in comfort and read a book long enough to decide I wanted to take it home with me. (Also the book shop makes money on the cafe and attracts customers in who want a coffee but have to pass lots of tempting books on the way.) Clearly in that case the bookshop is not objecting to people eating while fingering its merchandise.
I think it makes a difference also what sort of food. Hot savory/ pungent / meaty food could, I see, be off-putting. However, I live fairly near the sea and in our local shopping area often see people munching on local ice cream while browsing - it is a place popular with holiday makers, with a lot of shops that attract the tourist trade. These shopkeepers would be very unlikely to object, I think, to someone wandering delicately licking a vanilla cone. And while it is nice and comme il faut to sit down for a meal, ice cream cones seem designed to be eaten on the move, and I would be surprised if having an icecream in the street on holiday were seen as bad form.