It was his own personal “policy”, not some actual law or rule. He just didn’t want to accept large gifts that left him beholden to the gift-giver. He later returned the money to his colleagues despite their attempt to “work around” this personal policy. This is the rich people equivalent of having someone at work that’s well-liked but doesn’t like people to make a big deal out of it. He could use something to do his job more easily, but it’s kind of expensive so everyone pitches in a little bit to get it for him. If anything, the fact that such an apparently honest man who also went against the segregationist Southern Democrat party line at the risk of his own career to do the right thing stayed in office as long as he did is an argument against term limits.