Thanks for the page, I am happy to comment. :)

[Note: I am not a lawyer.]

The things you attribute to Linux and Bitcoin are also valid with Steem. No approval is required from Steemit to fork the repo, or to convince the legitimate witnesses to run your update.

The clause in the license is to ensure that any future development work is run on this blockchain, as defined by the witnesses (not Steemit Inc), so that the STEEM currency that we all hold is the beneficiary of changes. It's protection against confusing clones/altcoins, not forks. Any changes that the witnesses run is the valid chain and we have no say in that (other than as any other holder of stake).

As a staunch open source person myself, I too was initially surprised when I saw that nonstandard license, but it makes sense in a weird way. The opportunity to make changes to this code comes with the caveat that your subsequent improvements must benefit the specific community (STEEM holders) that provided that opportunity in the first place, which is why it needs to run on this chain. I see it much like the AGPL in that sense.