They do seem to be unnecessarily harsh on some artists/paintings, but as Canary says, it's what makes for popular TV. The viewer dictates what gets watched in numbers.
I'm unsure contestants would necessarily know what they were in store for them. It's been ages since I last found the time / enthusiasm to paint. Always one of those many many things I wanted to get properly trained in, but decades past, and I didn't get around to it. But were I an amateur and thought to have a bash, I think I'd expect constructive criticism and advice, but wouldn't expect my effort or self to be put down and disparaged / discouraged. After all, it's not easy to get a good representation especially of something moving around and a brief to show motion in a static painting.