Look, i don't need to argue with you.
Disconnect both posts on the battery if it pleases you.
For that matter, disconnect all of the batteries in all the vehicles around a one mile radius of you and unscrew all the light bulbs in the houses if that makes you feel safer.
I guess the FSM is wrong and my over 40 years of experience is flawed.
However, if the wiring in the car is flawed, there might be a capacitor that discharges when you do wiring jobs that is unrelated to the battery even being hooked up.
So, with your theory, if you disconnect the pos post and leave the neg post hooked up, the same thing could happen? Sparking at the terminals?
Okey dokey.
Disconnect the neg terminal at the battery, isolate it, then try to turn the starter over.
I bet it won't. I'll bet nothing else works on the car electrically either.
Same goes if you disconnect the pos post instead and isolate it.
Power has to come from somewhere to make a spark in the electrical system.
Either stored in a capacitor, or live from a power source..... (a battery)
Makes sense now?