Obviously, in the case of very young children there may even be the enclosed seat that holds a toddler in place, with leg holes and it wraps around the child's lower body. Hard swings are no longer considered acceptable due to the injuries sustained from impacts from them moving after a child leaves it. Once again, as with other playground equipment, kids are going to use swings in unintended ways, on their belly, standing up, doing other things that might likely end up in a fall impact injury. It's how children seem to learn things best, and parents hope these lessons aren't too overly painful or impactful to their overall health and wellbeing. You probably remember seeing how high you could swing, then fly off while at the highest point, to see how far you could go, in a competition with your friends. We've likely all been there as a child. It's also why those really long chains are no longer present. Could you imagine the safety zone those would require?

There's a particularly good chance the safety surface directly under the swing will get the most wear out of the entire playground, and it's also where a pad is recommended for loose-fill surfaces, so it isn't displaced so much that the geotextile fabric separating the surfacing from the sub-base isn't damaged. Then you'll have those rocks inside the safety  surfacing, which isn't a good thing, because it can also compromise the drainage system, and you really don't want to have to remove the surfacing to repair that. 

If it's a public playground, it probably a good idea to use a very heavy molded kick pad beneath the swing, that's heavy and hard for anyone to pilfer and remove easily. If it's a residential playground, you may be able to get away with a anti-fatigue mat at a fraction of the cost of one of the public playground kick pads. You know the ones with the squared open space honeycomb underneath them, that allow them to squish when standing upon them. These interlock well with rubber mulch, placing them about halfway within a 6" depth, so there's 3" of rubber mulch beneath the pad, and 3" above it. This tends to hold in place best, while still providing 3" of depth beneath the pad for great fall impact protection for the height of the swing, protecting amd preserving the drainage system and sub-base.