For any kind of a slip call, the safety onus is on the pilot, not the company. The pilot voluntarily enters a slip that says it's ok to call at 2am. The pilot voluntarily accepts the offer, and it's on the pilot to make sure they are safe to do it.

The only safety argument ALPA might have is with IA calls, and those are pretty rare now (until the gummster fire gets out of control in another couple months at least).

Should they be trying to avoid the 2am calls when possible? Absolutely! Arguing a safety/fatigue angle is a fool's errand though.