From the back of the title page of Modern Reloading, Second Edition:
"Load data was assembled from that published by suppliers of the powder with their permission"
I always look there for a starting place, but then usually go to the powder manufacturers site for final data.
As it says somewhere in the book, it is compiled for your convenience. Does anyone really think that any of the manufacturers would NOT want their data included in the Lee manual?
Education teaches you the rules, experience teaches you the exceptions (Plagiarized from Claude Clay)