Have I missed something on this? Are you disabled and need your DH's help getting round the shop? Or is he scared/useless at looking after his DC? Or doesn't he know one end of a shopping trolley from another? Because if none of the above apply I can't see why, as a few people have said, he or you couldn't stay in the car with DC and the other do the shopping?
We've done it that way since DS was new born and DS is now 8 years old. I call DH when I am ready and he pulls the car up in the pick up spot. Or better still, I leave DS and DH at home and get the shopping on my own (a bit of peace and quiet!)
If we have had to get everybody out one of us will pull the car to the pick up point, we unload and then the driver finds a parking space. DS doesn't get wet on the way to the entrance and the non-driver gets going on the shopping quicker.
Whilst I can see it's annoying when people without kids use the parking space, I think it's only a 'have a go at them' issue when there's only one of you with the kids. You don't have a choice on leaving the kids in the car then. But you did. So I can't see your problem to be honest.
Plus, you said you couldn't see a kid in sight. Did you check the car? Maybe they had a child, like yours, in a car seat tucked away. If it was rear facing like our old one was, you couldn't see the seat or the baby unless you peered in the window.
And, if they were packing their shopping up is it really too hard to ask if they were about to leave and wait for the space? Your DH presumably pulled out of the space you took up near them and zoomed into the space they left? If so, again, I can't see what the fuss was about.