Liability. If someone installs the battery or wiring harness incorrectly there's any number of things that could go wrong. You don't want to be sued over a battery fire if someone shortcuts wiring or venting things properly. There's a double standard in that you can buy a Ford 532 crate engine and it'll come with the wiring harness and fuel system with an "approved for off-road use only" disclaimer simply because people have been doing engine swaps and self repair since the dawn of the automobile.