The problem is not discrimination - the problem is that recruiters can do the dirty work of discrimination (or harassment or false advertising) and entirely shield these companies from lawsuit. The companies can simply claim ignorance to have known what was going on with the recruiting firms. The recruiting firms aren't big enough to sue. You wouldn't get anything out of them monetarily and you'd be unlikely to generate much publicity suing a recruiting firm that no one's heard about.
If we're going to move forward with this model where recruiters are run out of firms not associated with the company, the main company needs to be liable for any abuse on the part of the 3rd party recruiters they hire.