Voltage Reading On Voltage Regulator Heatsink
The whole thing is inside an enclosure, so the end user isn't supposed to touch the heat sink...

Sure, the manufacturer could have added some insulating silpad between the metal back of the power devices (which is connected to some voltage which will shock you) and the heatsink but...

It would have cost 5 cents extra!

Can't have that. Also, omitting the insulator gives better thermal transfer which results in a smaller heatsink. Thus it saves more than a few cents...

If there are 3 devices and they need to be insulated from each other, then you only need 2 silpads... and the third device just gets grease and makes the heat sink live.