Hardy0109 wrote: as long as it's appropriate
And that's what a dress code does, it sets a guideline as to what is appropriate.
Sometimes, people don't understand where the limits are and they need to be spelled out. People like to push the limits, and then suddenly you find that there are no limits. You may think that you have a good idea of what is appropriate, but my standard may be stricter (trust me, it is), and someone else may have looser standards and may think that they can even wear a swimsuit to dinner and it's all cool because they are on vacation. None of our standards are right, or wrong, but we as members of society need to be mindful of where they are appropriate to be worn.
Disney provides many different levels of dining that make it so that everyone can feel comfortable while still getting a meal. In a swimsuit you can eat at private dining, poolside, and even in some of the resort quick serves. In a t-shirt and shorts you can eat at just about any Disney restaurant except in the fine dining establishments. In resort casual attire (what the Disney Signature dress code actual falls under) you can eat anywhere except Victoria and Alberts. Don't even think of going to V&A without a suit coat or cocktail dress.
As a business, Disney gets to say what is and what is not appropriate on their property, and they do exercise that right frequently because people just don't understand what is and is not appropriate anymore. What Disney has done by creating dress codes is to say that a swimsuit, or a t-shirt and shorts is NOT appropriate in their signature restaurants. They have designated those spots as someplace that you need to look a little nicer.
As patrons of those establishments, it is our societal responsibility to comply with their wishes.