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 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.