1/ Yann Lecun probably is the one pushing for open source
2/ Mark isn't doing this for the greater good and for humanity. It helps his business because Llama is becoming a standard, and people are building / improving, which in turn helps Meta and Meta's business
They were basically forced into this by being so far behind that they'd need to rent OAI or Anthropic services to stay competitive, which at their scale would've probably cost them more than it takes to train llama and benefit from community research help.
At least that was the rationale behind the intentional leak of llama 1 back in the day according to some sources anyway.
1/ Yann Lecun probably is the one pushing for open source
2/ Mark isn't doing this for the greater good and for humanity. It helps his business because Llama is becoming a standard, and people are building / improving, which in turn helps Meta and Meta's business