From https://opensource.org/osd:

> 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.

A non-commercial clause is a discrimination against a field of endeavor and thus non-open-source. The license cannot restrict how the user is able to *use* the software and still be open source. There can however be requirements to distribute the source code when distributing the software, ala GPL.