We cannot state that a publisher has to use DRM as we do not have the closely-guarded facts about piracy.
"Piracy" when used as a reason for DRM is about as ethereal as an argument can be. We are given no real information other than a promise that the evil boogey man is being thwarted. This argument only holds sway if we assume that it is accurate. We know that pirates are not thwarted as evidenced by the fact that every form of DRM ever created has been cracked (usually within days) and we aren't even positive that pirates are the evil boogey men since we are never provided with any hard data that shows that what pirates do actually hurts us (they obviously hurt the publisher, no argument there).
EDIT: After re-reading my post I figured I should clarify that I am in no way defending piracy. It is an illegal practice that prevents people from being compensated for their work. However, my point still remains, we (the consumers) have not been shown that preventing piracy benefits us enough to warrant the inconveniences that we are forced to suffer (that pirates can easily avoid).
That is like saying people are going to shoplift from stores so why bother with cameras, security, or any actions to try and lessen the shoplifting. In the end, we the honest consumers end up paying for the actions of thieves. Companies, no mater what they sell have a responsibility to protect their products/investments/assets from thieves to the best of their ability.