| The anti-undead clause is both flavour and a safe guard so that you cannot gain an infinite number of surges via your Shadow Summoning. Theoretically you could summon the shadow, lose a few HPs, steal surges from it and then regain the HPs, then wash-rinse-repeat. At least one of the paragon paths will allow faster summoning of the shadow familiar and more familiars, so this could become abusive.
Plus the life is coming from somewhere in the basic healing ability, it's just a much more direct transfer - either your life to another or their life to you. This was intentional, particularly to allow the party to retain mobility with a necromancer as the only leader. If the life transference required both targets to be adjacent to the necromancer, it would keep the party very tightly formed and also very vulnerable to enemy artillery attacks. | 
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