His fist. And yes, usually holsters have a saftey strap to keep the firearm secure.
Notice from the pic his gun is secure, and a person would have to get right in his face to grab his gun. He'd probably already have his hand on it if some goon was approaching him in a threatening manner.
It's not as easy as you think to grap someone gun and start shooting either. Take yourself for instance. You aren't familiar with guns, so if you did manage to grab it, by the time you looked for and found the saftey, you'd be on ther ground spitting your teeth out.
I’ve actually seen it done before. They came up behind the guy, pinned his arms from behind while another guy relieved him of the pistol. The amusement park had asked him to leave due to a weapons policy (insurance). He refused. They just waited a while and lifted the pistol. He got escorted out to the police.
I was surprised how fast it went. Police took him away for trespassing. This was in the 70’s when you could do that to people. The guys who did it were both Vietnam vets so they weren’t antsy about guns.
I presume the gentleman is part of the Libertarian Free State Project in New Hampshire.