Well the assumption is based on the fact that Setanta is available in more homes in Dublin than in the rest of the country because of the way the basic cable packages work.
As for the Man U and Liverpool argument the difference is that there's a lot of Man U and Liverpool fans in London and the UK has a wider population spread.
Furthermore while Derry and Cork would attract interest from viewers other than their fans (one assumes their fans would already be interested) because of their league positions the likes of Bohs and Rovers can still draw a comparatively significant TV crowd in Setanta's target market (although not so much ABs but certainly C1s) which at present is heavily oriented around the Dublin area. This is despite the comparatively inferior displays by both these clubs this season.
I've already said there seems to be a bit of overkill with Shels but that's not their problem.