It _may_ be illegal where you live, but you have to remember that people live in other places than you, with different laws.
It's probably not illegal where they live judging by their tone. Often people who don't like something will tell you it's illegal to gain your compliance relying on your ignorance. They usually cuss and make a big show of it.
I'd be quite surprised if it was illegal anywhere. Dashcams are perfectly legal and even mandatory in some situations. Getting data from that footage is legal.
I guess the uploading of images done here might be murky, but again there's millions of dashcam vids and youtube and other video sites.