Children + Shoes = Nightmare for parents
Children scuff their shoes, that looks like what has happened here. So as they're not visibly falling apart, I think the shop assistant / manufacturer has every right to say that's wear and tear.
Whether you buy expensive trainers or cheap trainers the outcome will be the same. So it's motivation maybe not to go too crazy when buying shoes as boys in particular play sports, they scuff the shoes and so forth.
I know you wanted people to agree with you, but unless you can show a picture where something is hanging off, or the stitching is coming undone on an area where you wouldn't expect wear, then it's just one of them things.
An example, a friend got coaxed into spending a lot of money when they got a car, on some stuff that was said to repel dirt and stop scratches. Obviously quite expensive, obviously too good to be true. When they complained they were missold the product, they were told well if you read the warranty you have to do all these daft things, otherwise the car will gather muck.
Similar kind of concept, if your DC drags their feet on the ground they will scuff their shoes, whether a £10 pair or £100 pair.
You learn pretty quick that children and shoes/clothes plus playing / walking / dragging feet (particularly boys) will cause damage, so you go for the cheaper option which is just as good. You can still have shoes / trainers for best for going out in, just not playing sports / general day to day stuff.
Hope this helps.