Cell phone repair is tricky, but not even approaching impossible. As a former electronics tech (about 35 years ago), I was able to disassemble, repair, and re-assemble my Galaxy S4. It really was not difficult and, with a service manual, would have been trivial. Soldering and de-soldering surface-mount devices and tools to do so are required, but those are hardly impossible to obtain.
More importantly, the law is more about letting independent servicers, who are likely more skilled than I, to have the information and parts to repair the devices.
The battery argument is pretty bogus. It takes no service information to get a battery to over-heat. A service manual only reduces the risk.