[tzs]

> they may forbid using derivative works for commercial purposes

The most widely used definitions of “open source” do not allow such a prohibition.

[stanac]

Yup, if we take OSI as defacto authority on open source definition

> 6. No Discrimination Against Fields of Endeavor

> The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.

https://opensource.org/osd