I have four kids, and rarely take them out to eat. But I have on occasion, to family friendly places. So I can say with experience that it is *really* difficult to clean small crumbs off carpet without a vacuum or something along that sort, but it's very easy to get up quickly with a vacuum. If they have a tile floor, you can wet a napkin and sort of sweep everything into a pile and scoop it up, but you can't do that effectively on carpet. That amount of mess, while it looks like a lot, doesn't necessarily imply to me that a child was deliberately crumbling up their scone and dumping it on the floor, just that they were dropping crumbs when they took bites, and probably not holding over the table as well as they should have. In general I'm somewhat on the side of the moms here, at least based on that picture and the emphasis on the issue being the mess. I agree with the others, too, that if they were asked to leave *before* they were finished, how would they have made even a token effort to clean up the mess? They were told to get out. I'd be impressed if they could have gotten everything off the floor, but they might have made an effort to clean up if they'd been given time to do so.
I think that places that use carpet do need to keep in mind that it makes it very difficult for anybody without special tools to clean things off the floor. Dropped utensils, napkins, and trash, sure. But crumbs? Not so much.