The retail CDs say Not For ReDistribution and the OEM ones (the ones I have here are all Manufacturer supplied ones, rather than MS supplied OEM ones) say For Distribution with xxxx PCs only (xxxx=manufacturer's brand). Both have copyright notices as well...
Hence lack of clarity. I suspect I know what MS's lawyers would say though 
I know the discs you mean, I have a few here from various MS schemes I have been on in the past through work.
My current deal with MS is that I get a licence key for whatever product I decide to test, and I then download the ISO from them and burn it to disc myself.
As a result of this procedure, there is no warning on the disc itself about making illegal copies, etc.
The rules themselves don't say anything about not distributing the software to others, and that seems logical to me as whoever I give a copy to has to have a valid licence key to use whatever I give them anyway.
PS
MS lawyers = more slippery than a conga eel in a bucket of swarfega
