I assume it's mostly due to misunderstanding. It's been a long time since the open source definition was adopted and we're seeing more and more people now who don't know the history.
People who have been using a term their whole lives might not have any idea where it came from, or that it even has an official definition.
But where does the misunderstanding come from? Where are they getting the fraudster definition from? Do they see threads like this and think “I should be like that guy who has all those people yelling at him that he's wrong,” or is there some reservoir of memetic infection out there where this kind of thing goes unchallenged?
It's right there in the words of the term - it's not an unreasonable guess to think that 'open source' just means that the source code is 'open' in the sense of 'available', especially since proprietary software doesn't normally provide source code.
Compound this by the fact that many people are jumping into this field where English is not their first (or even second) language.
People who have been using a term their whole lives might not have any idea where it came from, or that it even has an official definition.