The message, including a 10000 EUR price, which is not mentioned on the page:

> Academic tradition requires you to cite works you base your article on. If you use programs that use GNU Parallel to process data for an article in a scientific publication, please cite:

> Tange, O. (2020, July 22). GNU Parallel 20200722 ('Privacy Shield'). Zenodo. https://doi.org/10.5281/zenodo.3956817

> This helps funding further development; AND IT WON'T COST YOU A CENT. If you pay 10000 EUR you should feel free to use GNU Parallel without citing.

This is strictly against the GPL [1] but somehow a chat with Richard Stallman made it ok.

[1] https://www.gnu.org/licenses/gpl-faq.en.html#RequireCitation

It is not "somehow ok". You either overlooked the last paragraph or decided to ignore it:

> "As the request for citation is not a legal requirement this is acceptable under GPLv3 and cleared with Richard M. Stallman himself. Thus it does not fall under this: https://www.gnu.org/licenses/gpl-faq.en.html#RequireCitation"

It is not the "chat with Richard Stallman" that made it OK. It is simply not a requirement, but a request for citation.