Basic blender went bad (motor ran but spindle wasn’t rotating). I wanted to disassemble to see if it could be repaired. Three of the four screws were Phillips head. I had to cut the casing open in order to discover why I couldn’t unscrew the fourth. It was a slotted spanner.
Seatbelts tend to be held in with Torx-head bolts. Right bicycle pedals have left-hand-threaded studs. Spanner-screws are a standard you’ll find drivers for in any good security or electronics/small-appliance repair set.
Odds aren’t that far off that this screw was chosen for their blenders decades ago when this screw-driver was more common, and this one part was never updated as the design … “evolved”.
i’ve got no problems with torx (it’s one of the best driver designs) and nothing wrong with left hand thread, as you said, it’s needed for bicycles, but using proprietary “security” bits is just, less than acceptable in the modern day and age.
Though i am inclined to agree with you on the design theory, it’s more than likely they have billions of those little proprietary things kicking around in a warehouse, and there’s just no reason for them to get rid of them.