Even if it is voluntary for Arabs, they wouldn't be drafted because they'd need to go against someone who potentially a relative. This is why all the non-Jews are voluntary, but the Army offers a lot for Druze and Bedouin when it comes to work opportunity in the army while they serve and when they are discharged into civilian life.
And if it's mandatory for Druze, why don't the women get drafted? Or do they do Sherut Leumi like religious girls? Do you know anything about that by any chance?