As I understand the situation, AA (and all US carriers) are responsible for the security checking of their passengers. As such, they want to ensure that their own staff are doing the ID validation and not relying on employees of their partner airlines for this task. By requiring that AA boarding passes are only issued by AA agents, they feel they more control over the checking for which they are legally responsible.

It really is a due dilligence process on AA's part. However, the other way is still done - AA will issue a BP for other OW airlines. Seems they trust themselves but not their partners.