I don’t get how the ethics of that are questionable. It’s not like they’re taking brains out of people and using them. It’s just cells that are not the same as a human brain. It’s like taking skin cells and using those for something. The brain is not just random neurons. It isn’t something special and magical.
Sure, we don’t know what makes us sapient or conscious. It isn’t a handful of neurons on a tray though. They’re significantly less conscious than your computer is.