I disagree. Like most design trade-offs, there are usually several considerations. I believe that having the door handle automatically disengage the deadbolt allows for more rapid egress from the cabin in the event of an emergency requiring rapid evacuation (e.g., fire). In my opinion, had the designers gone with your preferred solution (separate deadbolt release) you would create a situation where a panicked passenger could waste time trying to get out of the cabin by turnng the door handle before realizing that the deadbolt needed to be released in addition to turning the handle (note that critical thinking during emergencies is not always effective).
So, the trade-off is a toddler getting into the passageway vs. a simpler and quicker emergency evacuation. I tend to think that, in general, safety considerations have more weight in the decision making process than inconvenience. So, I think that they made this decision correctly.