Reputable sources (e.g. Haynes, Chilton's, Alldata, etc) are paying a licensing fee to republish OEM information, or at the very least asking permission.
Haynes is probably the only "paper" DIY repair manual source left, and typically they only publish information on older popular vehicles, where the income stream from the OEM manual is little or no interest to the OEM publisher.
While proprietary, the information is not that closely protected, especially on vehilces that are no longer produced by the OEM.
About 99% of automotive fasteners are NOT critically torqued, so any discussion about bolt grade is moot - as you suggested in your question.
The "original" information is derived from a collaboration between the OEM engineers, and the OE component manufacturers. So an injector hold-down torque is determined by the application, the materials, and the ultiate goal. Maybe that bolt could be a lot tighter, but not without cracking or distortion of the injector flange, crushing a seal, etc.
(For instance, Subaru knock sensor uses an M8 bolt, but is ultra-sensitive to the distortion created by overtightening. Final torque is a small fraction of what seems appropriate.)
The engineering information is addded to the OEM "repair manual" to aid technicians in replacing components. Often there are general torque rules, which apply when no torque is specifically mentioned. Specialized torque values are provided when the general rules don't apply (knock sensor, oil pan, transmission pan, timing cover, water pump, valve cover) and the desired torque is typically much lower than a blind assumption based on the general OEM rules or bolt size.
The information flows from the OEM design engineers TO the manufacturing line AND the "repair manual" ... "electronic database". In fact, part of the "design" of the new production vehicle would be developing the tooling to tighten injector hold-down bolts to the correct values. I can't speculate on the exact timing of the information flow, but I am certain the information is not "centralized" with the possible exception of OEM manufacturing enginnering oversight.