The kids who need help might not associate Kutcher with the organisation, but donors and supporters sure as hell will.
At this point, he has written a letter of support, lauding the moral upstanding of a convicted rapist. Any way you slice it, it’s not a good look from a fundraising perspective. Why would I support the anti-sex trafficking group that has a rape apologist as it’s figurehead? The nuance of writing a letter to support a friend who has done terrible things is it’s own murky issue. But from a purely business standpoint, Kutcher is a liability for the moment.
Your example ignores the trajectory of the situation. If Kutcher had supported the rapist, received backlash, and joined the org, it would be an individual taking steps to make better choices. As it stands, he is this organisation to many people (I couldn’t name anyone else involved). His actions reflect directly on the org, and ignoring his actions is not a good look.