I attended a picnic in a park today with a group of friends from church.
They were quite amused by my insistence on keeping my picnic blanket a shoe-free zone. In fairness to me, they did acknowledge that my picnic blanket was a lot nicer and more comfy. The others had blankets made of rough wool, while mine was a soft, fluffy blanket.
It did get stepped on quite a few times by some of the energetic children, but those people who sat on it with me did remove their shoes or flip flops.
A picnic blanket is for eating off. You don't put your shoes on the kitchen table and you shouldn't put your shoes on a picnic blanket.