The natural inclination of scholars to want to share their insights, as well as, increasingly, the pressure on scholars to produce in greater volume, creates a natural opening for unethical publishers to exploit authors. Although the problem has grown since the advent of online-only publishing, it is not a new one, nor is it limited to open access publishing. In fact, questionable publishers exist on both sides of the open-access/toll-access (subscription) line.
The best defense is simply for authors to be critical of "too good to be true" publication offers. Check out Think, Check, Submit's video on choosing the right journal for your research:
We offer a variety of tips for assessing a publisher's quality.
You may also be interested in some other ideas on assessing a solicitation to submit to a specific publication, or this article about identifying high-quality, sustainable OA publishers from Duke's Kevin Smith.