I don't know about "many people", but I have some examples. The popular book, The Architecture of Open Source Applications absolutely refused to change its title to "The Architecture of Free and Open Source Applications" when I requested it.
There are people who think that "free software" is the loony bin and absolutely refuse to be associated to it. There are examples of that in this HN thread. There are also people who call themselves "free software" and disagree with the FSF, although they don't disagree on what "free software" means. OpenBSD is a clear example (look at their release song lyrics).
I understand "Open Source", I understand "Free Software", but to be honest, I always thought the "Free and Open Source Software" (FOSS) term is just stupid. We have a local saying: it's like trying to reconcile both the goat and the cabbage.
They were quite clear that they did not want to be associated to free software and were very annoyed and unreceptive to the idea. I can't find the exchange now, but it definitely happened. I think it was over email.
You can so the experiment yourself and go to open source projects and ask to go with gpl for future contributions and to put free software into he readme :)
The tone in "Many people" and "go out of their way to make sure" makes it sounds quite different.
Almost as if the majority of developers were actively refusing to touch GPL code.
I've been working with many developers and I can say with confidence that the large majority has only mild preferences and does not refuse to use or send patches to a project simply due to licensing.
Asking people to change license of an existing project is quite demanding and takes more effort that doing nothing which makes it an unfair comparison.
Citation needed.