The ability to put Taunt where you want it is worth more than an extra health.
It's not. The strenght of Taunt depends on the state of the game. For example if you face an opponent that wants to trade minions anyways, Taunt is far less effective. That's why on good cards (virtually every Taunt that still gets played after it's rotation to the Wild format) Taunt is worth less than 0.5 Mana. It is indeed also worth 0 Mana on the Lackey, because the Lackey also gives +2 Health, but:
This card has Vanilla stats (=Attack+Health-1/2) and also gives 2 Health, so I think it is better than the Lackey in the early stages of the game, whereas the Lackey becomes better as soon as bigger minions enter play.
Titanic Lackey = 0.5 Mana (1/1)+1 Mana (+2 Health)+Taunt= 1.5 Mana and Taunt
Beaming Sidekick = 1 Mana (1/2)+1 Mana (+2 Health)= 2 Mana
So one card cheats 0.5 Mana and Taunt, the other cheats 1 Mana.