Lower back pain is very common and most people are trying to exercise their muscles in an effort to reduce this pain. The challenge is that some exercises are good and some are just downright bad.
The reason these bad exercises are still hanging around is because some people don't have the same back as you. You might have a degenerated disc and that particular person in the gym doing the "wrong" exercise doesn't have a degenerated disc. You may look at the exercises he's doing and think, "maybe I'll try those."
Another situation is that the damage that guy in the gym is doing to himself hasn't happened yet. There's a paricular exercise where you are seated and you push and twist your lower spine against resistence. This is the exact mechanism that CAUSES herniated discs. In this case, while it looks like the person is not having any trouble at the moment, they are suffering from what's called "yet" disease. They don't appear to have any pain "yet."
Here's a recent video I made regarding this subject.