Why are you under the impression the Germans did not bomb British postions? A first hand account of the evacuation called Sword of Bone by Anthony Rhodes that I am familiar with made it fairly clear they were being bombed and strafed all the time. They were not without air protection during the evacuation though so it was not a free for all. You have to remember it's really not very far for airfields in Southern England to fly to Dunkirk and provide air cover. I am sure that helped mitigate German attacks to an extent but there were still plenty of attacks.

It was Hitler who stopped the German Divisions to overrun Dunkirk for several days until it was too late, this is often explained as an intent to show restrain to the British and looking for a negotiated peace to cease hostilities. This was likely to have ample concessions for Germany in the Continent while leaving the UK free hand in the Oceans and its colonies. However as already explained here the Luftwaffe was authorized to continue its pressure on the British positions and transports.