I'm not related to [university], so ignore this if you prefer only internal accounts. ;) This is just my opinion. One possible pedagogical motivation for such a structure is as follow. With the end in mind, there are very few jobs which privilege holders of bachelors level studies in either, Ph, Po, or Ec.
(i) If you were to demonstrate depth of specialisation, to work as say, a bioethicist at a hospital (Ph), a political analyst in a think tank (Po), or an economist at an asset manager (Ec)... then at the same price-point, all bachelors job applicants would be out-competed "on paper" by early/mid-career masters degree holders, and creamed by early/mid-career doctoral degree holders. (Of course in practice, the more years of working experience, the more the candidate might demand, and then you know, the less qualified applicants might provide better raw-value, but let's stick with this simplifying assumption for a bit.)
(ii) A bachelors degree holder in these areas with extraordinary extra-curricular abilities (e.g. math, sales, presentation ability, project management, what have you), is also going to win competitive commercial job-hunts mainly on account of the extra-curricular qualifications, and not by the nature of their coursework. (This doesn't apply to mediocre competitions which "value the major as a class-marker," rather than as a commercially competitive advantage.)Zooming out for a second, let's consider an imaginary advanced club containing genetically and socially engineered superpeople who ALL have PhDs in either Ph, Po, or Ec. What might the significant competitive factors be, within each discipline? (So assuming Philosophers compete only among each other, and Political Scientists compete only among each other, and Economists, ditto.)
If the point of a PhD is to linearise the competition along the axis of one discipline, and so a set of PhDs is really a set of specialists, who have passed muster among the society of peers in that discipline. This is usually mediated in publications: "daring to make a point, and defending it, at least in form" - it's basically a communications test, within the domain specific language (DSL)of the discipline. This is a really useful practice in curating an elite class of peers oriented about each specific discipline/DSL - it's great for deciding who gets to work in universities, as the people are the brand (what economics IS, is determined by WHO practices economics at any point in time), and the brand equity of each department is pretty much what universities have come to compete against each other on.
However, when you back to the demands of competitive COMMERCE, the hyperspecialisation of a PhD is in effect, often though not always, overkill. Now we extend point (ii) above, in the context of what matters in a commercial setting...
(iii) Within the pool of ultraspecialists, a competition for who gets hired to join an army that seeks to slaughter competitors is going to depend strongly on whether the ultraspecialist has any other skills (hard, soft, social, technical, what have you). Erase from your minds the notion that PhDs are like superweapons which are highly protected assets which win battles and wars - these game-changing individuals are far fewer, than the total number of PhDs on the market - it turns out that having a PhD is actually pretty common within certain circles. So we're playing at a whole different level now, seeking to differentiate among a lot of candidates who "look the same on paper"... and often, to cut this thing short... three to four years of pre-specialisation in interdisciplinary studies builds a better foundation for such competitive candidates... than specialisation (at the doctoral level) built on specialisation (at the bachelor's level).
For further reading, consider:
The illustrated guide to a Ph.D.
