I don't think it is the case that physical violence generally is okay in video games. For example, if you learned that someone played very realistic torture simulators you might feel uncomfortable about that, or about leaving your children with them.

I think the idea is that guns and bombs and such are cool, and probably lots of people have the impulse to just let loose some destructive volley, blow something up, shoot stuff, run people over, etc - and video games let them do that without consequence. If you saw someone playing GTA and they were running over drug dealers and having shoot outs with the police - that seems like a pretty wild and fun fantasy. Conversely, if they murdered an NPC, then found that NPC's grieving family members and kept dragging the murdered NPC's body in front of them to make them depressed - that would be more atypical.

In other words - if the fantasy you're indulging is being powerful (guns, bombs, using the car destructively) - then that's pretty normal. If the fantasy you're indulging is sadistic, that's pretty abnormal.