Doesn't SIP cause issues with the TiVoization clause? I guess I'm not an expert on this, but it's possible Apple wants to leave the possibility open of making SIP mandatory.
An OS with mandatory SIP will be the point I stop upgrading. I'm fine with optional features like this, but the instant it becomes mandatory and can't be bypassed I'm gone.
The decision of not shipping GPLv3 user space tools predates SIP by several years, far more than I would bet SIP has been in development. I think it's just a legal issue as in lawyers don't want to touch it period