You only get to choose your government every n years, so as part of their pitch to you, they make a promise (in law) that their officials will not be bribable - any subsequent bribe violates that promise and that law. By contrast, you can choose which restaurants you visit (on a day-to-day basis), so they have no need to make such formal promises or contracts, so they don't. They are then free to offer service x for price y, you are free to accept or reject, and they are free to change it the next day.