The Miami Heat is an organization with strict rules regarding its players' conditioning, which applies to the offseason. They may be at the extreme of the spectrum, but the expectation of staying in shape the entire year is understood for all top professional athletes.
Some teams have transparent rules everyone knows about and has to abide by. The Chicago Bulls in the 90s had Michael Jordan and Scottie Pippen. That's your alternative – the best players on the team set a standard that everyone has to follow, and if they don't, there are consequences (via The Jump):
"Me and Michael set a standard that if you come in like that, you are going to be ridiculed the whole season for being overweight and you're gonna get punished for it some way or another. We're gonna use some verbal punishment as well. "
Scottie Pippen, via The Jump
Both ways have their benefits. If you have the Miami Heat approach, you cover the scenario in which your star isn't crazy about hitting the gym and working on conditioning. Still, then that's another friction point in your relationship with the locker room.
If you are like the Bulls or the Spurs, for instance, and your best player leads and puts pressure on the rest of the team, you can coach that person hard, and everyone else falls in line. That has the added benefit of internal policing – players are more likely to accept criticism from a teammate than a coach or GM.