You gotta think on the grander scheme of things. Personal ties shouldn't put someone above the law and that is the dilemma Garp faced on Marineford and Kizaru has faced just now. The ones they wanted to protect are criminals and if they show any form of leniency, the only ones left believing that justice can be upheld would waver. I hate that this happened with Garp and how Kizaru handled his "Ace" moment since he's the one who personally executed Stella but I also understand that they are prioritizing the order side of things. Like it or not, the world still needs the Marines since not every pirates are like the Straw Hats. They are menaces across all seas that even Eustass Kid, a guy we were teaming up with an arc ago, was a good riddance when it comes to public safety (he willfully involves civilians in his fights and does not care about innocent casualties)
Which is also why I believe the Navy has respect for both Shanks and Whitebeard, the only Emperors who are actively protecting territories, are not plundering countries and are not actively opposing the government.