I would wager most programmers implementing AMP don’t feel they have a choice. Either they need to do it for SEO purposes or they need to do it because project managers say so it for SEO purposes. That’s kind of the whole reason AMP is bad.
I understand people can contribute with their eyes open; that's fine. People volunteer bug reports (for e.g.) to paid software all the time, open source or not. My criticism was actually of Google soliciting those contributions in the first place.
"Open source" is just a useful fiction in this case: what would a fork of AMP even do? It does however make the project corporately palatable to others that might want to implement it (i.e. Microsoft/Bing), plus it attracts unpaid contributions to fix the work of Google's highly paid engineers ("Building the future web, together."). The former might be necessary but the latter is extremely problematic IMO.
Google could have set up AMP as an independent W3C group (for e.g.) at arms length from its competitive interests, but it didn't, and recent AMP product announcements show us why.
I think you have a rather odd idea of what open source means.
Community ownership and independent standards committees are not a requirement of open source. As long as forking is allowed, as a project owner, you're within your rights to veto any change. Consider Python ("Benevolent Dictator for Life") and Linux. Or for a smaller project, consider Elm.
It's not unusual for employees and volunteers to work together. Furthermore, the "volunteers" are often employees of a different company.
But as per GP's point, what would forking the project do? Who other than Google is positioned to host an AMP fork, and why would they choose to do so?
The centralized nature of the project makes it hard to understand what the point of it being open-source is, apart from being good PR and getting free work done on it.
