Even if this is true - and I don't know that it is, but for the sake of this post, let's assume it's true - it's irrelevant. The covering is not as it should be. Therefore, the shoes are flawed, and presumably not acceptable to you.
Let us say I buy a new car, and upon taking delivery I notice that the front right fender is a different shade of blue than is the rest of the car.
I inform the dealer that there is a problem with my new car. That the front right fender is medium blue, and not dark blue like the rest of the car.
He replies that, "The color of the paint on the fender is simply a matter of aesthetics. That it's the wrong color will not affect the longevity or quality of your car."
What he has said is absolutely true. The longevity and quality of my car would not be affected, were the fender medium blue, hot pink, or purple with green polka dots.
But is this a reasonable response, which ought to settle the matter?
Of course not!
It's a flaw - even if only an aesthetic flaw. It's a legitimate reason for me to be dissatisfied. I would be entitled to a refund, a replacement car, a re-painting of the existing vehicle, or basically some sort of mutually acceptable resolution to the problem.
Seems to me, if you purchased a pair of new shoes with turn out to be aesthetically flawed, and you were not informed by the seller prior to purchase that they might be aesthetically flawed, you have a legitimate reason to be dissatisfied with the shoes.
Aesthetic flaws matter!
Demand an exchange or a refund.
Or decide that you don't really care about the flaw, and stop worrying about it.
Uh, that's just a channeled sole. The covering for most shallow channels like that one- which are pretty much purely aesthetic and don't affect wear- wears off pretty quickly with normal wear to the sole. It's normal and nothing to be upset about. You might not want to see it on a new shoe, but you're going to have much more significant aesthetic damage to the sole the moment you step outside.