I think what the league wants to address is more of the tanking than the load management, although both are some concern. If a team is managing the load of some of their key players, at least that still means that they are trying to win.
As to the Celtics, Tatum clearly twisted his ankle recently in a game. Brown got kneed in the back by LeBron. I don't think the league should have any issue with them being load managed for a period of time as a result. As others have said, there is an age clause in the new formal rules so they can do whatever they want with Horford.
And Porzingis has a long history of injury, as do many players with his dimensions/size. His load is clearly being managed as it should. The actual rules were posted by someone somewhere, and I can't remember if Porzingis qualifies as a "star" or what specifically applies to him. But there is nothing wrong with the Celtics "managing" his load through the season. It is not his fault or an indication of his toughness or competitiveness that his body is not as durable as other players. Nearly all "super-bigs" in the history of the game have had the same issue. There should be a "super-big" clause in the rules