Your logic is correct, there is no real direct monetary vested interest in keeping their guests on the property. It comes down to more of a legal liability issue. If a guest gets hurt on or off the property, there is a strong chance they will be sued. To keep their insurance liability rates down, it is in their best interest in trying to "oversee" as much of their guests' stay as possible. This is why they recommend staying on the property.
This is also why they offer their own tour packages using their own drivers to attractions that they feel meet all their requirements.
When a guest leaves the property on their own, they are usually warned that if something occurs it is not their responsibility. But that doesnt prevent that guest from suing them if an incident happens off the property. Although the guest loses the lawsuit since they did leave the property on their own, the AI's have "deep pockets" and lawyers know this, which is why the AI's do what they do.