This was more an oversight by the developers, I think. Most every quest was designed with the mindset that it would never need to be cancelled.. Unfortunately as the game has grown and more quests have been introduced, it's steadily become apparent that there are exceptions where cancelling a quest (like this one) would be very convenient to the player. It all just boils down to a poor design choice early on, and it's something they probably can't easily change now globally. (Although it wouldn't be too difficult to change this on a case-by-case basis, by adding a cancel option to the NPC's of the offending quests.)