Is it acceptable to release a paid application to interface with an open-source backend?

Question

I'm currently in development of an application that consists of two parts: PHP files that a user uploads to their own server, and a smartphone application.

Because the PHP files are going to reside on that person's server, I thought it would be beneficial to everyone that they might as well be open-sourced (Mozilla liscense or something of that effect). These PHP files will extract data from the site, which can be read by anything that interfaces with it.

However, I intend to make an application that will read and edit this data, which will be a paid application and closed source.

Is this legal/ethical/a bad idea?
(Apologies for any mistakes, this is my first post on this exchange.)

Answer 1

(I am not a lawyer, this is not legal advice, etc.)

My understanding says that this is perfectly fine. A copyright license is a license from the copyright holder granting specific permissions under specific conditions. It is up to the copyright holder what to license, to whom, under what conditions. It is even common for the copyright holder to offer different people different licenses to the same code - that's called dual licensing.

In this case you might even be forced to it. For example you might choose to make your server code under the http://www.gnu.org/licenses/agpl-3.0.html so that you can improve your odds of getting back any improvements in your server code. But as examples like http://www.fsf.org/news/2010-05-app-store-compliance make clear, you cannot use that license on the itunes App Store.