OK, the window one was a bad example because it effects other people, unlike ear defenders.
A better analogy might be sunglasses on a bright day. Why is it OK to block out light, when it feels to much for our eyes but not OK to block out sound when it feels to much for our ears?
The linking of ear defenders with disability aids is a social construct, there's absolutely no reason non-disabled people can't wear them (I'd prioritise those who need them most, but they're cheap and easily available, so not usually an issue).
Lots of things that have been developed to help with disabilities have gone on to benefit everyone. Eg. Lifts, audiobooks, voice recognition software, electric toothbrushes. It's called the cut curb effect. Necessity breads invention, so for those able to manage without, there is ofter no incentive to look for anything better. However once a solution has been found for those who weren't managing, the potential benefits for everyone can be seen.
In this case the DD doesn't 'need' the ear defenders, but has found she is more comfortable and able to concentrate with them, so why denie her that benefit just because disabled people also benefit from the same thing?