I am of the opinion that both reward systems and worldbuilding should actively be designed to restrict player freedom. Complete freedom for player is not desirable and an active impediment to play. When everything can be done, then every option is equally valid and none any better than any other. In a sandbox, you need restrictions to enable the players to make choices.
Those restrictions don't have to be hard barriers, though some may be. But they should be repellant, pushing the players' paths back into the center space of the campaign when they start drifting towards the edges. People love picking from options much more than imagining completely new things in an empty space, and it leads to much more fluid play.
Or in another to put it, maybe overly poetically, the game structure and worldbuilding of the setting doesn't have to restrict the possible answers, but it should at least narrow down and specify the questions. Let the players freely choose their methods, but provide some structure for what goals are viable and what tools available.