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.
The other half of the "every piece of mail costs money" coin is to send back every prepaid envelope. What you put in it is up to you, but the thicker and heavier it is the better.
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.