It's unfortunate that this seems to be a thing, but a thing it appears to be. I've been suspicious of it for some time now. I've even fallen victim to it once.
One of the comments has been that if you're nice to sound guys then they're nice to you but unfortunately this hasn't always been the case for me.
I arrived at a summer festival gig where we were supporting another band. I went to the sound desk straight away, introduced myself, spent time talking to them both and asked how THEY wanted to do things. We set out a plan of attack but I gave them the lead and I was polite at all times.
We set up and did a very long sound check, fine-tuned everything and it was sounding perfect, I was delighted with the sound and they were really helpful and took time to pay attention to the details.
We started the show, and they'd changed everything! It was muddy, could hardly hear the vocals, the kick drum was non-existent, and it had previously been perfect. I was asking for things to be tweaked between songs over the mic, the sound guys wouldn't make eye contact with me just kept giving the thumbs up, but no noticeable difference was made to the sound while we were playing.
The main act came on and guess what? Absolutely pristine sound. I went straight to the sound desk to ask what had happened, and their reply was 'welcome to the music industry.'
After I had been dragged away by the rest of the band and calmed down, I ended up feeling really disappointed that there were people who would deliberately sabotage a live performance because we weren't the 'main band.' I was polite, friendly and helpful to them from the word go but they screwed our set up regardless without batting an eyelid. Not only is this unfair on the support band and the audience, but surely this is unprofessional in the extreme?