I went backpacking with some friends when I was in my early 30s, and one night we camped by a lake that had snow on the opposite shore. We decided to go take a quick dip in it. We knew it would be way too cold to enjoy, we just thought it would be fun to briefly experience it just to see.
I'm guessing it's like that. Why do people climb the tallest mountains? It's not fun and they don't get anything for it, they just want to say they were able to.
I wouldn't do either of those things today, but I feel like I'd be a hypocrite for making fun of someone who does.
Water that cold will kill you pretty quickly, too. So it depends on if these folks are just getting out of their car for a bit or if they're going for a hike. I saw that the one guy was running, but it wasn't clear of that was just a "I wanted to see how it felt" thing or he really thought a fun was a good idea.