If the tabs are, in fact, connected to the middle (collector) pin, and you had two transistors with their collectors connected to the same thing, could you use a heatsink as an electrical connection? I'm assuming that'd be... bad practice to say the least.
Very bad practice... If it's not in Murphy's Law, it should be...
When you want it to isolate, it conducts. When you want it to conduct, it isolates.
Initially, it will conduct of course (to a degree determined by how well the screw breaches the oxide).
When it heats up and cools down (alu and steel expands and contracts at different rates), even the best screw connection will get oxidized. Aluminum oxide is an isolator.
And by reading this (the expanding and contracting part), you hereby swear to never mount a transistor to a heatsink without a suitable spring washer under the screw, at the peril of burned parts, right?