I am a bit surprised that you're disappointed that it does exactly what you told it to - people usually have the opposite complaint.
If you're using it interactively and watching what it changes, I'd trigger the tests when you think it's needed. And if you want to go more hands-off, why not add try to encode the same nuance you'd use into the rule?
But yes, anything you want to ensure really needs to be a hook.
edit: realizing with "precommit" you probably meant a git hook not one in their harness. I'd have written the same response more or less though. :)
But then you could just be storing up a lot of problems…
I read that this is because it wastes time looking through past conversations and other context to figure one what you might want it to do - a less ambiguous prompt would be better.