If you can provide a reference to BSA documentation supporting this definition, then I will gladly pass it on to the leaders in our unit. However, I will refrain from enforcing any such a rule as I believe that would go against the intent of the uniform. Per the insignia guide, the uniform is intended to be a medium to display "what each youth or adult member has accomplished with program opportunities...". I fully encourage scouts/leaders to wear a complete uniform, but I would much rather a scout/scouter wear a 90% correct unform than get hung up on such details and discourage them in the process. I tend to apply Mark's statement that "No Scout is turned away or publicly admonished because he or she isn’t wearing the right thing."
Without evidence to the contrary, I assume the term "non-uniform wear" allows such pin/awards be worn without a uniform, essentially overriding the following rule. "It is the responsibility of all leaders of the Boy Scouts of America and especially of all commissioned officers and chartered councils to cooperate with the Boy Scouts of America in preventing the use of the official uniforms by those who are not registered and in good standing."
Finally, there are a number of items that are allowed to be worn on the uniform that are not explicitly approved in the insignia guide. I see no difference with the mentor pins. Here are just a few examples:
Wood Badge beads. They are explicitly allowed to be worn on the Sea Scout Uniform (which is typically cleaner than other uniforms) but no mention is made of them for other uniforms.
Belt fob and camping beads. These are sold by the scout shop and are worn by many troops but are not mentioned anywhere in the insignia guide.
Several lodges have pocket patch sets that cover the entire pocket instead of just the pocket flap (as approved by insignia guide).
The insignia guide outlines that "all members" are to wear nameplates with white text on black plastic. No other reference to nameplates or nametags is made. However, commissioners wear red nameplates, the scout shop also sells leather nametags, and many wood badgers I know wear wooden laser cut nametags. Not to mention the countless scouts/leaders that do not wear nameplates.
In summary, a lot is left up to interpretation. I have no issue if someone wears something that they earned and supports scouting ideals.