If they use a proprietary third party library, they might not have a source distribution license of said library. The same way that you can't just unilaterally

If they use a proprietary third party library, they might not have a source distribution license of said library. The same way that you can't just unilaterally decalare an MIT licensed project is GPL without ensuring that all of the code was dual licensed or covered under a CLA. EDIT: I saw another comment that says that it's "just" the GUI applications for Windows and Mac that aren't open, but the core functionality is. I know that in Windows-land, it's _very_ common to use proprietary UI libraries like Telerik UI[0], or devexpress [1] [0] https://www.telerik.com/purchase/individual/winforms.aspx https://www.telerik.com/purchase/individual/winforms.aspx [1] https://www.devexpress.com/ https://www.devexpress.com/