Your point really can't be argued, as it does state what is best for the turtle. However, many people have to compromise the turtles needs with their needs or limitations...such as space available or money.
Although I'm not arguing, something to think about... Birds have very large areas that they live in, and travel a lot...however, nobody ever complains that it's inhumane to keep them in your house, much less in a small cage.
Personally, I think that water quality, diet and other issues can be more important than the size of your tank, providing that it is reasonabley large. I'd never consider even one adult RES in a 30 gallon tank to be reasonable.