Hardware Keyboards, Synths or ROMplers and natural instruments are typically allowed to be resampled and freely distributed
I'm pretty sure that this is incorrect. Any recording (e.g., sample) is implicitly copyrighted in the US and most other countries. Any re-recording of those recordings (e.g., by sampling a digital piano) is subject to the original copyright. I seriously doubt that it's "typically allowed" to sample a digital piano and distribute the samples.
Instruments that are not based on samples are not covered by copyright protection. So, most analog hardware keyboards and synths (even digital synths when not using samples), are fine to sample and distribute, as are natural instruments or electromechanical instruments.
Can anyone back up the Rules statement? Is it guesswork or is it based on solid legal advice?
IANAL, but my father was an intellectual properties attorney and we'd discussed these issues, and I've studied material at copyright.us.gov . This is also often discussed among sound designers and the general agreement is that it's risky to distribute samples of instruments that are covered by copyrights.
It sure would be nice if I could freely distribute my samples of the Yamaha S90ES piano, Ensoniq MR76 piano, and a number of Yamaha CP4 pianos and epianos. But I think I would be violating Yamaha's copyrights. (I have these samples so that I can use them in my DAW, which is way more convenient than having to always use the hardware keyboard.)