Here's my question: Is it legal to expose tv series from web sites on a mobile platform? Would the developer(s) be sued for creating such a software ? Or since the data is already exposed online(i.e web sites where users can watch their tv series online), it would be considered legal to expose the data on a non-commercial or commercial platform ?
Would be better if instead of the actual video, it would be the URL (link to the ressource) ?
Addressing the question of programmer liability, generally speaking, a person who creates something that others use to infringe on copyright is not liable is liable if there are non-infringing uses for the thing (Sony). They can be liable if they intentionally contribute to infringement, viz Grokster
one who distributes a device with the object of promoting its use to infringe copyright, as shown by clear expression or other affirmative steps taken to foster infringement, is liable for the resulting acts of infringement by third parties.
End-user liability is different. In order to legally download (watch) anything, you have to have permission from the rights-holder, and from the site operator. The site operator will have terms of service according to which you can access material: if you violate those terms, you are infringing on copyright (you didn't do what the permission to copy required of you). If the site-owner knowingly distributes material without permission, he is an infringer and you have no legal license to copy (watch) that content, so you too are an infringer. If the site-owner unknowingly distributed material without permission and generally complies with the DMCA safe harbor requirements, he is not an infringer but you the user are. Congress has not created an innocent-infringement defense where you can defend yourself against an accusation of infringement by proving that you didn't know that the material was distributed without permission (there is an option to reduce liability for statutory damages to $200 in case of unknowing infringement, but not make all liability go away).
In the United States (and likely in other jurisdictions) the copyright holder has the exclusive right of "public performance" of the work, which includes exclusive control over broadcast and digital presentation of the work.
See Aereo for an example of a U.S. company that tried to challenge this exclusive right in a digital context and lost: Aereo captured over-the-air television broadcasts and presented them via an online service. The Supreme Court ruled that this violated the exclusive public performance rights of the copyright holders of the broadcasted works.
The work's availability as free of charge from the copyright holder is not relevant, as we can see in the above case. Aereo lost, and they were repurposing works physically broadcast for consumption by anyone who could pick up the broadcast signal.
By contrast, if you want to use a link to point people to the location of a video on the Internet that has been legally offered by the copyright holder, there is no law stopping you from doing so. Also, you might be able to serve a webpage that uses an iframe to frame the original source (though the case law in this area is somewhat conflicting), since your page that includes the HTML <iframe src="..."> is in no way a derivative of the framed page. However, you could cause a trademark violation if you surround the framed page with your own branding and confuse the true origin of the video.
If the video you are linking to is an obvious case of infringement, posted without permission of the copyright holder (e.g. the latest Game of Thrones episode hosted on pirateallthemovies.foo), then systematically helping people find these sorts of illegally-posted resources can make your liable for contributory infringement.
Whether or not merely distributing such an app (versus using such an app) is illegal depends largely on whether the app automatically performs copyright infringement for its users and whether the app has substantial non-infringing uses.
