This holds true until you begin looking at bigger picture market economics, then it could be argued that "proprietary OSS" is harming "true free software" by preying on less principled developers (and users).
Oh, please. Non-GPL OSS developer here. I'm not some weak doe in the woods being preyed on by evil companies and their MIT and BSD licensed software. I know exactly why I strike the bargain I do - its because I want to be able to make money selling proprietary extensions and modifications on top of any software I write. And I want to encourage any developers using software I write to do the same.
I use BSD/ISC/MIT licenses because I support the right of other developers to make modifications to my software and keep their modifications closed source if they want. I support developers feeding their families. And more $ usually means more ideas, more developers and healthier ecosystem.
Get off your high horse. I'm not a "less principled developer". I just have different principles thank you. I think having developers who pay rent writing OSS is a great deal for everyone.