According to Wikipedia's separate article for plastic bullets, they were developed to be less lethal than rubber (no citations for it though). IMHO, this could teach a useful lesson, that even the ones intended to harm less can be made harmful to the point of deadliness. I think it depends on the (ab)user, even dihydrogen monoxide is deadly when abused.
Just wanted to "[point] out some consideration that hadn't previously been mentioned" and "[give a tiny bit] more information about the topic" (https://news.ycombinator.com/newswelcome.) :/