The best evidence to support the statement that in the late 1800s a law made it easier for magazine companies to do business is: "This was a boom time for magazine publishing, owing part to the developments in offset printing technology as well as to the Postal Act of 1879, which had granted magazines discounted mailing rates."
This quote directly mentions the Postal Act of 1879, highlighting how it provided discounted mailing rates, which facilitated magazine publishing during that time.