Because they want market share, and throwing a hissyfit over being asked to add an "I am over 18" checkbox is not good PR. If Debian starts refusing to work in California because it doesn't want to add a checkbox, it will simply be replaced by someone who adds that checkbox and doesn't throw the fit.

I disagree slightly. It may not be good business, but it could be good PR, situationally. I expect a lot of 2nd-tier distros will refuse to implement it, and see a boost in their installs as a result.

Debian, Ubuntu, etc., they'll all fall right in line because the clear and immediate losses will outweigh any PR issue.

As the article says, it's not about just checking a box:

"Every OS provider must then: provide an interface at account setup collecting a birth date or age, and expose a real-time API that broadcasts the user's age bracket (under 13, 13 to 15, 16 to 17, 18+) to any application running on the system."