The topic is odd to me. There is a variety of choices that could be made, as well as extra options. The only real reason that comes to head is that they don't have an idea of which power sources will be available later. They wanted to keep it open.
That being said, imagine if Power Sources were part of the monster stat block. There could be certain feats and powers that provide bonuses against certain types of power sources. I don't immunity would be the way to go. You have Arcane, which could have Dispel Magic. You could have Martial, which could counter other Martial attacks. Power Sources could act as a springboard for new mechanics and therefor, making a difference on mechanics instead of just being descriptors.
That being said, imagine if Power Sources were part of the monster stat block. There could be certain feats and powers that provide bonuses against certain types of power sources. I don't immunity would be the way to go. You have Arcane, which could have Dispel Magic. You could have Martial, which could counter other Martial attacks. Power Sources could act as a springboard for new mechanics and therefor, making a difference on mechanics instead of just being descriptors.