A practical example: Assume that baby talk is unwelcome

People who express opinions on that matter almost universally say that they don’t like being addressed in cutesy baby talk voices.

Despite this, many people talk to non-verbal adults in baby talk voices, on the assumption that it’s ok because they’re low functioning, or have a young mental age, or something. 

This is bad. Being unable to say no does not imply consent. And if someone can’t say no (or has been taught not to), it’s important to be especially careful to avoid doing things to them that they are likely to dislike. 

Since you know that most adults dislike baby talk, you shouldn’t use baby talk voices with adults. Especially if they are adults who can’t talk or who usually aren’t listened to.

Edited to add: Except adults who like baby talk and have made it clear that they prefer it in some contexts. People are allowed to have unusual preferences.