The rotors are clamped to the wheel hub by the lug nuts when the wheel is mounted.
I think the screws holding the rotors to the hub are just there so rotor doesn't wobble when you're doing a runout test without the wheel mounted.
(just use a screw extractor and some penetrating oil to back the broken off screws out)
I wonder why those screws are breaking. (screws too tight and rotors turn slightly to engage the lug studs = snap off the screw heads?)