The sons wanted to save their brother WB didnt force anyone, they come to fight for ace as they all loved him
That's literally the point of whole arc. Ace thought since he is son of roger he didnt deserve to be loved but when he saw people are willing to sacrifice their lives over him and how much loved he is he cried out of joy even tho people were going to die saving him
Thats the whole point of marineford and Ace, one of oda's best and most beautiful characters to this day, sadly his dumbass death ruins all of this