It's the CIV4 vs TW mindset.


In civ 4 when you have a artillery unit, of any kind, you roll up next to the city on campaign map and hit bombard, which then destroy the implacements, citizens, and surrounding areas.

However you never actually zoom in to invade the city, you just charge some units in against the garrison on the campaign map, as all fighting since Civ 3 is done on the campaign map.

In TW you zoom in for battles so there is no need for any military actions, (other then blockades) to occur on the campaign map.