I don't think there's any doubt that entire teams keep track of Drouin whenever he's on the ice. He's a guy you gameplan for.
The fact that it WAS Drouin might actually explain Erne's thinking. As in, maybe there was a bit of pressure on him, if only from himself, to make sure he played just as physically against Drouin as all the other Mooseheads. That would at least make it understandable if he got caught up in the moment and didn't want it to look like he was taking it easy on Drouin, rather than being focused, as he should have been, on the fact he was about to hit somebody from behind. Just a thought. Obviously there's no way to know what he was thinking.
The fact that it WAS Drouin might actually explain Erne's thinking. As in, maybe there was a bit of pressure on him, if only from himself, to make sure he played just as physically against Drouin as all the other Mooseheads. That would at least make it understandable if he got caught up in the moment and didn't want it to look like he was taking it easy on Drouin, rather than being focused, as he should have been, on the fact he was about to hit somebody from behind. Just a thought. Obviously there's no way to know what he was thinking.