Having a monopoly isn't illegal. Using monopoly powers in certain ways is, and this isn't one of those ways.
And yes, companies who have to survive in a competitive marketplace drive away non-paying customers all the time. Because they're not paying, and so the slight chance of turning them into paying customers, or benefiting existing paying customers, is worth the loss.
And yes, companies who have to survive in a competitive marketplace drive away non-paying customers all the time. Because they're not paying, and so the slight chance of turning them into paying customers, or benefiting existing paying customers, is worth the loss.