In my experience, a lot of staffing agencies do this. And it's not that the jobs aren't legit, or that the companies they work with are shady. Sometimes the staffing agency is the only way to get in the door, especially with small to medium sized companies that don't have a ton of HR staff themselves.
One issue, though, is that sometimes staffing agencies will try to claim that a company is a "client", when, actually, they're reading the same want ads you are, submitting your resume, and then trying to collect a placement fee. Employers don't love that, and if an employer has a choice between a staffing agency candidate and a non-staffing agency candidate, they are likely to go for the candidate they don't have to pay a fee to hire.
I've gotten plenty of jobs through agencies, though, and sometimes there's no avoiding them, so:
1. Submit your resume, go in and talk to them, and be nice to their staff.
2. Call them every day or so and ask what's out there.
3. Be very clear about getting the name of every company they submit your resume to. Don't duplicate their efforts, because it muddies the issue of whether or not the agency is owed a fee, and that can kill a job offer.
4. Keep using your own networks and your own research to look for work.
5. Don't hesitate to speak to more than one agency, and be honest about that. You are happy to speak to anyone who might help you find a good job. You can't afford to limit your options in the market by being exclusive to one staffing agency, even if you had a way to tell agency ads from direct employer ads.