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.