The Steem logo up there in the top left is public domain. If anyone can reuse the code on any other chain, someone could start a brand new blockchain unrelated to our community with the same logo, call it something defamatory or confusingly similar, and dilute the market and cause significant confusion/uncertainty. It's a protection against clones, not forks (of the actual witness-defined Steem Blockchain).
It's important that any improvements and updates on this codebase go into this community that built the base, don't you think? It's sort of like the AGPL in that sense, in my opinion. If you make changes, you have to return those changes to the community that provided you the opportunity. The community is the collected stakeholders of STEEM.
If you like AGPL so much, then switch to that. Problem solved.
Returning improvements is totally fair. Restricting people's ability to use the code as they see fit (and that includes with a different chain) makes the code less useful and discourages people from wanting to work on it or be involved with it.
Make the Steem logo and and the brand a registered trademark. That would prevent anyone from using your brand as theirs. The logo itself could remain in public domain (license) and be a registered trademark at the same time.