Just curious why people can travel "freely" in open jeep on safari?

Because to be able to have a great view to the animals and natural settings you use open jeep for game drives and safaris. The drivers and guides can interpret animal behavior and they can act accordingly in case there is any kind of danger. In some settings you have to sign a waiver in case anything happens.

uhm, because they have an elephant gun sitting next to them lol.

Maybe it's because it's in the wild that you don't need to sign a waiver.