I once was at a concert where one of the openers was a completely different genre of music - poor planning on the concert planners part - and the audience was MEAN. Their music was good, I liked it just fine because I'm one of those people who likes a lot of different genres, but it was a hard rock/alternative concert and they were a folk singer with an acoustic guitar. You could tell they were nervous because the audience was calling out mean comments and booing. I was disappointed in the audience, tbh.
So, the band would probably care if the audience booed at them, even if they were paid!
That is poor planning for sure. Perhaps if there was a 50/50 split, there could have been some cheers too, but normally, it is the negative voice that is the loudest.
I was at a show kind of like that too. It was collective soul and they had a singer like that open for them. I remember people being pretty decent though. That's too bad they had to act like that for your show.