Why are sourcerers imprisoned in Fort Joy?

Why are source users incarcerated on the Fort Joy island? I could not find in game answers ( they probably are there, but the game is so huge ). I do not understand why Divine Order put such expenses to transport prisoners and maintain the island instead of simply disposing them after being captured?

Edit: I am just at the end of Act II, if possible please avoid spoilers for ActIII

In prologue and act 1 it was actually mentioned, on surface, sourcerers are gathered and source muted to protect everyone, prevent attracting monster(voidwoken).

But we do see in act 1 sourcerers are being experimented on and transformed to silent monks.

You will discover more as you progress in the game.