I had a similar problem this morning. A studenty type in a floppy hat darted into the last remaining P&C space right in front of me. I was already hopped up about a separate parking-related issue, so as he sprung merrily from his car, I rather leaped from mine.
"How many children have you got in there, then?" I asked him.
"None," he replied, "but I've got a bad foot if that helps."
"There's a vast sea of empty disabled spaces," I pointed out. "Why don't you use one of those?"
Well, his foot was evidently not bad enough to prevent him from executing a J-turn and screaming off across the car park like Ray Doyle... watched by a biggish woman and her doughy, sour-faced mother, who had just arrived, sans children, to load up the car in the very next space.
"I haven't got any kids, either," remarked the biggish woman. At which point I'm afraid I said a naughty word.
The question of whether stores should provide P&C spaces is entirely irrelevant. They do provide them, and as a rule they mark them clearly enough that there's no excuse for anyone else (or at least anyone able-bodied) using them - particularly when there are other regular-sized spaces available.
However, if there are no free disabled spaces and someone who would otherwise be entitled needs to spill over, then no, YANBU.