I would imagine that folks are giving the spinach to chickens that are in a run all of the time. Or, just in the winter. That's when I give spinach.
Pretty much this - we just got out of winter, all the grass and greens in my run are gone now, and spinach is something that we eat that the chickens also enjoy. I give them very little, just bits and pieces I end up with after washing spinach for dinner.
Spinach interferes with calcium absorption, so chickens should only get it in moderation.