If I develop QGIS plugins for my company do I have to make them available to others? I use QT-Designer and think that it also depends on the question how you develop your plugins, right?
For example: If I'd work for public authorities they could be totally fine with publishing my plugins as there is no competition between them and because you don't have to gain a high ROI.
But: If I work for a private business that has a lot of competitors than my CEO could have a problem if I'd tell him that I develop a plugin for QGIS that every rival firm could use without contributing to the expenses we had for developing the plugin. I indeed think that it is the duty of companies who use QGIS to contribute time or money to the QGIS project itself in order to keep the project "healthy" but this must be made apart from sharing plugin sourcecode in many cases.