Performance tuning (horsepower) and bypassing emissions shouldn't be possible by the approved technicians anyway. From my experience, those capabilities are reserved for an engineering tool, not a diagnostics/repair tool. So, those don't seem like legit arguments to me.
One safety concern for untrained technicians is the possibility of activating motors, and other circuits during the repair process without ensuring those things are safe to operate in their repair/disassembled state. So, there is a
legit safety argument, but that's true for many tools that are sold at the local hardware store.
Fair enough, although not for most vehicles going forward -- they're encrypting all of the bus traffic and have made other security improvements (in response to the 2015 defcon Jeep hack that demonstrated how weak vehicle bus security is among most automakers).
One safety concern for untrained technicians is the possibility of activating motors, and other circuits during the repair process without ensuring those things are safe to operate in their repair/disassembled state. So, there is a legit safety argument, but that's true for many tools that are sold at the local hardware store.