lazyness on the part of burnside and whoever runs bitfunder (I think i've heard the name Ukyo(?) or something?)
Keep in mind in order to do it they have to come up with an absolutely fool-proof security system with digital signatures, etc, otherwise all kinds of shenanigans could be played.
Why do you assume it should be the exchanges who create arbitrage mechanisms? They are in competition with each other, not to mention that very few related securities are actually run by the same entity on both exchanges.
In this case, the issuer on Bitfunder is the same as the pass through operator on BTCT, so it should be up to Ken to create this tool should he wish to help even out the pricing.
Arbitrage cannot take place if the shares can't move - the entire point of arbitrage is that the "good" is being bought one place for cheap and sold at another for a higher price. Its possible that this could benefit both exchanges.