Well, running the engine (or not to) with empty AdBlue tank has nothing to do with topping it up yourself.

- You can let the tank go empty and not turn up to the workshop for the fill up, how does disallowing to top it up yourself prevent that?

- Many current cars with AdBlue will flash increasingly annoying warnings at you, and although many people drive with check engine lights lit, running out of urea will display you a counter of remaining kilometers and the car will simply refuse to start when empty. Again, nothing to do with whether you push the car to the dealership on foot or fill the tank yourself from the bottle (and actually, there are very foolproof anti-spill bottles, which let the liquid go only when tightly screwed onto the DEF tank).