Operative word in your description is, "Should."
I readily admit to having your sensibilities, but I have come to learn that the Dress Code is more suggestion than a hard-n-fast rule that cannot be bent at Disney.
Think of it this way - folks spend the day in the Parks and want a nice dinner to end their day. Should folks wearing Disney t-shirts then be turned away for wearing their Camp Clothes to dinner?
And if a Disney t-shirt gets accepted, then so too would other t-shirts.
Are leather sandals really any better than flip-flops? Not really, so flip-flops enter dining rooms.
Disney suggests a dress code, but they seem to have an unwillingness to police the matter.
And as much as Camp Clothes at Signature restaurants might bother both you & me, I can also understand, and accept, Disney's position.
That being said - we shower after the Parks and put on 'proper attire' before enjoying a Signature meal, but I can understand why others are unable to do the same. The result - we expect less than a fine dining experience at Disney Signatures - because it is Disney - that welcome children and their child-like adult-parents to be children (like wearing a Halloween costume to a Signature meal) as well.