Is it really a bug though? If you shoot something with an arrow, it is kindof benificial/makes some sense to beable run up and take your arrow back. I'd say shooting a giant monster with an arrow, then running up grabbing the arrow and shotting it with the same arrow again would definitely be an experience building excercise

. But yeah, its not that hard, but neither is just beating them to a pulp with your fists. More like a lack of level building activities than a bug.
Rather than make the few monsters that you can gain experience from harder to gain experience from, Id much rather just see more monsters, or maybe a few more 'meaningful' experience building skills. Stealing arrows seems like less of a cheat than say, disarming your own runes, which is really the only other experience building, agility based skill there is. If you do away with stealing arrows from lords, you may aswell just go ahead and do away with the theif character class.
As a matter of fact, why not just do away with it anyhow? I'm not sure there is a single person who plays kingdoms that has the larger portion of their overall experience in agility. Or who steals for any purpose other than gaining general level and raising their maximum hp.