The forces generated by this airflow are pretty weak. They are only sufficient to pull light, thin curtains inward. That explains why people with heavy plastic curtains typically don't have this problem. Also, if someone has poor water pressure or a poorly atomizing showerhead, they may not see the curtain suck in.
What is the best way to keep the curtain from sucking in? Because the force is pretty weak, the easiest thing to do is to sew weights in the bottom. Or, if you have a metal tub, magnets can hold the curtain in place.