These fonts are shared under Free, Libre and Open Source licenses, which allow anyone to use them, modify their design, contribute more glyphs or styles to their non-nuclear families, build upon them and redistribute them further.
They are generously published for free, feeding an ecosystem of sharing and collaborations.
To support the designers, you can plan in the design budget of your project some money to donate to the designer directly, commission them with custom font designs, invite them to give a lecture and spread the word about their great designs!