So if I plug a dumb device into it that only makes electrical contact, it won't make that negotiation.
So, so long as Apple's MacBook USB charger is compliant, it won't destroy anything USB that I plug into it, so long as those devices are also compliant. Right?
A "dumb" device that doesn't negotiate will get the minimum 5V@100mA that the USB spec allows, dedicated chargers often decide to not drop the power after the negotiation window is over but a proper host device (laptop, etc.) will drop all power if a device fails to negotiate.
So yeah, if you plug something into the Macbook charger it will either not charge (dunno if it supports traditional USB power specs since it's a type-C only charger) or charge as any other charger already does.
Re: "dumb" devices, this is how it's supposed to work, yes, but I don't think I've ever seen a charger that won't happily dump 1A into a resistor wired over the power pins - including brand name (Apple) chargers and yes, the USB ports on a MacBook Pro. I've never seen a modern device supply only 100 mA.
For devices that DO attempt to negotiate, yes in those cases usually you'll see it work according to spec.
I haven't tried USB-C yet, it would be interesting to see if devices have gotten stricter.
So, so long as Apple's MacBook USB charger is compliant, it won't destroy anything USB that I plug into it, so long as those devices are also compliant. Right?