Because the empty throne oath is one big fat lie
And the Gorosei keep a lot more secrets asay from him
As they said the Marined are merely the outside face of the WG. Akainu obeys but evem he should have a limit at some point and a breaking point, he's fleet admiral of the marines, a new burden will be placed on his shoulders with the whole picture out.
If he is unhinged even after all that at very least many of the marines will be affected, might even divide the marines as a whole. Fujitora, Garp, Sengoku, Tsuru you name it among the big shots.