But ultimately, Ashton should know that already. I will grant him that the statements written by him and Mila were given months before all the details were made public.

I think this is the most likely factor in why both Kutcher and Kunis wrote statements. They knew a friend was in court (who probably told them it was all crap and that they were innocent) and that their friend would benefit from a reference from someone well known who could vouch for their good character. So they did it to help a friend who they likely believed would be cleared of wrongdoing.

Then all the details came out and it became clear that he was probably guilty and statements were already sent. They could have requested them back but probably thought it wouldn’t make a difference in the result of the case and either didn’t worry about it for that reason or just tried to rationalize that they were standing by a friend because of the good old days when he wasn’t doing terrible things.