There is a wiring harness in that rubber boot, but chances of them "puncturing" a wire or "artery" is low. Usually there is a very thick rubber over the wiring harness to prevent abrasion with the firewall and keep out water, dirt, bugs.
This was prob the easiest for them to do, but if it was just for 12v power they could have just tap into the fuse box inside the car... Yes it could have been laziness too because they did not "look", you do not need much experience for installation, but experience helps with projects like this
I have done this before a couple reasons, the rubber grommet keeps out the water and dirt, there's already a hole in the firewall so no drilling just piercing the rubber to make a hole, no drilling into a plastic cover in the firewall then using a rubber gasket to prevent abrasion, or I did not have a grommet to go over a firewall opening.