I was recently looking into my family history, and doing some research on my great grandfather's rural grocery store in 1930's Kansas. I found an old newspaper

I was recently looking into my family history, and doing some research on my great grandfather's rural grocery store in 1930's Kansas. I found an old newspaper ad from the time where the four local stores were excited to offer a weekly sales program where each one would put a product on sale every week, and the other three would agree not to compete on that product. "Huh, that's an interesting gimmick... wait, wait, WHAT? How was that legal?"