i had a friend back-in-the-day that was a "recruiter" and this is how he explained it:
essentially, they give the shytty jobs to minorities.
he explained that the reason they do that is because businesses that receive "bad" candidates won't go to an agency or a particular recruiter if they (agency/recruiter) continue to send the businesses minority candidates.
i asked the dude why he would ever participate in such a thing and he just rubbed his fingers together (made the cash motion).
you'll know if they are fukking you over if they never call you back or hardly stay in contact with you or give you jobs that you didn't ask for or consistently place you in positions for which you are over qualified or not related to your field.
yes, i do have first hand experience with that type of treatment.