Blown Coverages - Who do we blame?!

[Defensewins]

I have a big problem with way our defensive backfield plays zone defense. I am so tired of seeing our defenders release a receiver at the most critical time of the play to cover an empty area (zone). During a typical pass play the Qb releases the ball around the 3 to 6 second mark.
When playing zone a defender should never release defender after 4 seconds, after 4 seconds coverage should revert to man coverage, because the Qb is about to throw it or already has. Our defenders prioritize more covering the field then covering a receiver. That goes against everything I have ever been taught in sports.
Even when I played organized basketball or soccer you start out playing zone but you do not release a guy unless another defender is there to take him. That especially applies the closer you are to your own goal that you are defending. That is basic stuff our team does not do. Why? Coaching or player that do not get it? Or both?