Children should be wearing shoes at a playground or at any other public place. Who knows what is on the ground or what they can step on. Also, I install these mats and can tell you that they are very expensive. They do cut down on injuries and lessen the shock to young bones and joints.
The reason they are black is, I would assume, is aesthetic, mainly because they are not intended to ever be seen. I have never seen just black mats. We always put a light mulch over them usually 4 inches but dependent mainly on the type of mulch used. I think the smart schools, playgrounds etc. just use mulch and nothing else. In my experience, sand is a no-no. It quickly becomes cat poo and where there are kids, they are going to throw sand. The school is quickly filled with kids with eye injuries, sand and cat poo. :)
Oh, so in otherwords, you are one of those Liberals.
Kids throwing sand is bad right? Never mind kids falling or landing in it at the bottom of a slide, after all, a cat may have poo'd in it. (not likely, cats knew better to hang around playgrounds where evil little kids might swing it by the tail and launch it into the wading pool)
Of course Liberal nanny's can never fathom the thought little johnny can be so cruel.