It might be. I wasn't told to do it in my very limited training in behaviorism, but I didn't progress very far before I realized I hated behaviorism with a passion. (To clarify - I don't hate the idea of using systematic rewards and punishments to alter behavior, but the idea that you can create any behavior you want by doing this, and that you don't need to consider the target's internal states at all, just their response to stimuli.)
It might also just be an outgrowth of viewing autistic people like very young children. The speech style you describe is known as 'parentese' by linguists, and it's instinctively used to talk to babies and toddlers (who love the way parentese sounds, and find it useful in learning to understand speech). However, it's often inappropriately applied to older children, people with developmental disabilities, people with dementia, etc, and many of those targets find it annoying and offensive. I think it tends to be used when someone sees their target as in some way being similar to a very young child.