When you go to a restaurant, you have to pay for the meal – there’s a quid pro quo. But you don’t have to leave a tip (we’re leaving aside situations where you have a large party and they automatically add 18%). A tip you leave because you want to note and appreciate the service you received. Maybe a tip is expected, but a waiter can’t sue you for not leaving one.
So too with bribes, gratutities, and law makers. If a member of Congress makes a deal with you where you’ll give him $10,000 in exchange for voting for your favorite bill, that’s a bribe. But if he votes for your favorite bill and then you send him $10,000 because you’re excited about his vote, that’s a gratuity.
As the Supreme Court has said,