I'm carrying over my own post from the ALPB to here from a discussion about why part15 is allowed at all. I think this is very interesting...
To answer the question of why part15 radio is allowed.. If you go back to the original part15 rule [part24].. I found it interesting the way this new rule was conveyed; In 1938, it was foremost a "substantive" rule, IE: a public right to freely utilize. However it also emphasized an "exception" of not causing interference to licensed stations.
In essence the rules haven't really changed in the last 80 years, however now days the interference exception seems to be the foremost aspect of the rule
What's also interesting is what the catalyst had been which actually had directly prompted the rule to be written, which is something I never heard before till a day or two ago.. As discovered in a 1942 QST magazine article, it was a remote control invention of Philco Electronics that could travel 75 feet. I posted about this early yesterday morning in my blog: part15lab.blogspot.com/2017/03/thanks-philco-for-part-15-broadcasting.html