The solution needs to be cultural, not legal. The culture change needs to come bottom up within the company. Employees in those companies need to advocate for

The solution needs to be cultural, not legal. The culture change needs to come bottom up within the company. Employees in those companies need to advocate for active contribution in open-source, not just in a parasitic form. This can come in many forms, building groups/guilds to advocate open source, mentioning it in one-to-one, setting expectations around open-source in job interviews, and requesting time and money to contributing to open source. People need to stand up and say "this is not okay", and stand up for what they believe in. The only way things will change is if the cost of contributing to open source is lesser than the cost of losing talent because of it.