From now on, Microsoft will ban applications in the Store that use open source software and ask for money for it. When software is generally available elsewhere for free, developers are therefore not allowed to charge for it, unless it is their own open source software.
The new rules restrict a practice in which developers can earn money with little effort from software that is basically free. Of course, inventors of open source software can charge money for an application that uses that software, although in that case unreasonably expensive prices are not allowed. In practice, it is not yet clear to what extent Microsoft can enforce the new guidelines, unless it concerns an obvious clone of an application.
This happened recently, for example, with sound editing program Audacity. The developer of that open source program was forced to release a Microsoft Store version of the app because paid clones were offered in the virtual store. Aside from the fact that others could monetize otherwise free software, many clones wouldn’t work properly either.