Auto brake isn't just for "inattentive" drivers. It's also for self proclaimed "attentive" drivers that can still lose focus at a bad instant.
A turning car can still slam in their brakes when they're not completely out of your lane. Its rare, so your maneuver will be fine most of the time. But sometimes other drivers do unexpected things.
I don't think anyone would complain about these systems if they had all of the context and made good decisions. But it is a problem when either humans or automated systems stand on the brakes based solely on what is in front, without regard for sides or rear.
I do believe these systems are a net win. But they have rough edges that at least cause a lot of near-accidents because they don't behave the way drivers expect each other to.
My car auto-brakes at 5mph for parked cars that are backed into spaces, which I am sure reflects poorly on my driving in some way, if we're adopting the "bad things only happen to bad people" model.
I've also had it activate in normal driving due to someone else doing something dangerous. I'm pretty sure I was on top of those situations but I'm happy the system had my back even if I didn't need it.