Why doesn't chainmail ruin a cast iron's seasoning but metal scouring pad will?
Steel wool, or other scouring pads, are made of quite small, rougher metal fibers or ribbons that will get into much smaller spaces, and clean away much smaller bits of matter, which includes the "seasoning" that coats your pan and/or the metal surface itself.

In contrast, the chainmail links are much larger, and each link's surface is relatively smooth overall.  This is great for removing bits of food and other foreign matter, but does not abrade on the much smaller level would be required to do permanent damage to the seasoning and/or the metal of the pan.

See these close up views of magnified steel wool and magnified titanium chainmail.  While the chainmail pictured is not steel, the smoothness would be rather similar between the two.  As you can see, the surface of the two materials is much different.  The smoother surface of the chainmail would therefore do far less to erode the surface of the pan, while still being quite effective at cleaning away food particulates.