This crystal ball is about 6 inches in diameter. While touching it, you can cast the scrying spell (save DC 17) with it.
While scrying with the crystal ball, you have truesight with a radius of 120 feet centered on the spell’s sensor.
Notes: Sense: Truesight, Scrying, Detection
Is there no charges? In our campaign we basically had unlimited scrying because there was no limit described
Perhaps they are relying on the built-in limits of the scrying spell? It only lasts ten minutes, then you would have to cast it again. That would mean another save, and a successful save means no more scrying on that target for 24 hours. Law of averages says you’re not going to get too much scrying on a single target before they get lucky and cut you off.
My understanding is that that's why it's such a high rarity magic item.