Pentasa is a better drug for UC than it is for Crohn's - for the reasons you mentioned: Works only on the surface layer of the gut and does not address the deep-seated inflammation of Crohn's. And the more recent publications have shown that this drug is really no better than placebo at treating Crohn's. But neverthless, for many docs it remains a commonly prescribed first drug to try - the first step in the "step-up" therapeutic process of tryng the cheaper, milder drugs first before moving up to the stronger and more expensive ones. So in that sense it is "normal," if perhaps regrettable, to be put on Pentasa.
More modern "top down" therapy of giving strong drugs first shows that you get a better outcome in the long run compared to the "step-up" approach. but unfortunately not all docs, and especially not all insurance companies, are on board with that.