At first I thought it was weird and off-mission, too, but then I learned more about the particular cryptocurrency they affiliated themselves with: https://www.stellar.org

It made more sense then. Stellar XLM and the mission of the Stellar Foundation seemed more in line with the altruistic (though perhaps naive, hence this whole topic) motivations that Keybase publicly espoused. Although, obviously, it did not pan out in the end as far as Keybase is concerned.

I still pay my kid's allowance in the XLM cryptocurrency I got from their "spacedrop" promo though :-D