Some movies are available for free online because they are public domain movies. Copyright movies may be available online at the discretion of the production company that owns the copyright.
Some production companies choose to make some of their movies available for viewing free online and generate revenue by including advertisements with the movies. Since the company owning the copyright decides which movies are included in the online selection, some titles are excluded.
Public domain movies are also available free online, as they were never released with a copyright notice or the company holding the copyright chose to let it expire. Most production companies choose to renew the copyrights of their films, therefore most films cannot be viewed for free online, as it would violate copyright law.