The Evil binding is... well, it's almost timid and polite. Evil through and through, to be sure! Foreign and offensive to Heaven, though its Law outweighs. It's a small enough matter to shrug off, here in one of Heaven's safe places.
However, the armies of Heaven are filled with volunteers, and those volunteers are Lawful and Good outsiders.
When a young soldier is taught about the ways of wizards and their bindings, it isn't told sternly that it would be desertion to acquiesce, because its chain of command isn't fearful of any loss of discipline in the ranks.
It's taught accurate information, about the relevant properties of the magic, about the potential risk, and the potential reward, about how Heaven notices and accounts for the rare gaps in the ranks that may result, how the magic is warded against especially in Heaven's safer places, how one can take further precautions personally if one cares to.
It's taught elsewhere, too, what sorts of initiative are accounted for, and what would damage the ability of the Heavenly host to coordinate and marshal its resources.
So, as a practical matter, if a somewhat less young soldier is the target of an Evil binding it could choose to throw off, it may also choose to accept. Heaven may be diminished forever if it meets its end in some wizard's lair, but Heaven will not be diminished by keeping its own outsiders as slave-soldiers, and Heaven will not be diminished by foreclosing useful opportunities to advance the Good simply for being high-variance.