If you take someone into your custody via an arrest, you are then responsible for their wellbeing. This is common sense.
Imagine if someone is in the middle of a shallow river getting arrested. Would it be okay to hold their head underwater while you are restraining them? Of course not.
The officer is responsible for making sure that the suspect is not put into harms way during the course of having them in their custody. If they fail that very basic expectation, they should not be in law enforcement period.