Some applications made by commercial companies employ a paid/community model for their software, meaning that you can use the open-source community version with less features, or you can purchase a license to use the paid version, which is based on the community version and includes more features. (Think IntelliJ IDEA Community vs IntelliJ IDEA Ultimate)I am wondering that if the source code for the community version is freely available and licensed, why don't users just fork the community version and implement the ultimate features on their own?
For applications using the paid/community model, why not just fork?
licensing;multi licensing
The paid features can sometimes be burdened by patents or other legal issues which makes it impossible to clone them as free software. But the value in the paid version is often not just the additional features. The paid versions often come with a support contract. You can of course often also get a separate support contract for the community version, but that contract will rarely cover any issues with the 3rd party modules which replicate the functionality of the paid version.Sometimes the license conditions of the paid version allows stuff the community version does not. For example, make and distribute modified versions which don't need to be open source.The paid version might have various certifications the community version does not. From a technical standpoint this might sound of questionable value. After all, if the paid version is certified to be compatible with system  XY and/or compliant with government regulation ZY-343-2012, then the same will apply to the completely identical community version. But there are certain industry sectors where such formalities are mandatory for one reason or another. Paying the paid version can be cheaper than getting your own certification for your deployment of the community version.