It is an industry trade group for organizations that use direct mail marketing. It gets mixed reviews, and it's not perfect, but it seems to be basically legitimate.

With email spam, costs are so negligible that there's no way it's worth the trouble of removing an address. Every piece of physical mail costs money for printing and postage, so while it's not a big incentive, a cost saving incentive actually does exist.

When I moved several years ago my new home got lots of catalogs for places I'd never shop at. I was shocked at how significantly it dropped the volume. Did nothing for things local ads, but the thick glossy catalogs all stopped.