PaulG definition: "Good business practice" - spending money on an activity that raises more money than it costs.
Whether you read your mail or throw it away is irrelevant.
The fact is that the NRA isn't sending a mailing to you. They are sending a mass mailing to millions of people. Sure, each mailing costs money. But even if most people throw them away, the contributions they receive as a direct result of the mailing pays for the cost of the mailing and adds needed funds to the coffers.
It may be irritating but it works.
This is not any different from the mailings I get from Home Depot, Lowes, Giant Foods or Safeway Stores. By the way, I almost always throw these away along with much of the NRA mail.