Having a FLAC (WAV) is a privilege, because it's hard to get, not just for individuals, but even for companies.
You need to buy or fetch somehow a CD and grab the WAV from it. That is whole lot of work plus money invested. And that is the reason there are so many fake FLAC files around.
People who bought fake FLACs are frustrated, some don't know it's a fake (mp3 really) and believe there is no difference. Some are frustrated because they can't get what they want in FLAC, so they console themselves it's the same sound anyway.
And of course, there are people who do not pay attention or do not care, and people with bad hearing.
Any one of them may become 'FLAC hater'.

 

I guess.. no, wait. I'm sure I was just only clarifying the origin of fake up-scaling which literally can't apply for the fake flac case.
You're welcome