You attribute too much to management, as if everything not expressly forbidden is omitted for a reason.
Corporations do not have 100% visibility into what their employees are doing in totality and they often don't write any rules for or against something until after an employee does something so bad it creates negative PR.
There's no employee handbook rule that tells me I can comment on HackerNews, or tells me that I can't (well, I hope not). So until I get a call from Google HR telling me I did something bad on HN, I do it.
I don't think this is Google specific, I don't think the vast majority of employees in the tech sector obtain permission for both after hours contributions they make on the internet, or even stuff they do while at work, even though most companies have a policy not to use work computers for personal things.
People just don't compartmentalize their lives like that. Like say you're at the office, you're browsing some GitHub repo on a project completely unrelated to your company, and you notice an easy to fix bug, and send in a PR on your work desktop while at the office. Is your company allowing you to do this explicitly, or do they just not care, as long as you're being responsible and not spending all day at the office working on non-work stuff and not deliverables.
The COVID era brings this even more into focus, with everyone working from home now, really, how many people are doing other work during office hours?
You're trying too hard to attribute master planning to management, when the truth is, most of these tech companies are more chaotic and autonomous internally then most people realize.