I know it has been said before but the only way I can think that it could not go over the fence is if smokey is tethered under the island or something to that effect only has a certin range and the fence was built at that line... Smokey could not cross over the top because "his leash" didn't reach far enough..

Just some thoughts I know I read by someone else about the tether

__________________
Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.