"Does not apply to operating systems under terms that permit a recipient to ... modify the software without restriction"

That sounds like it doesn't even apply to most open-source licenses, since they usually do have some restrictions, like not being able to change the license without permission of all authors, or removing authors' credits, plus you have to display the license to the user etc., IANAL but perhaps those could all be interpreted as "restrictions" that make it not eligible for exemption.