Why does Life Tap scale with spell damage?
The simple answer is that spell damage improves your spells, Life Tap is a spell, and thus spell damage should improve Life Tap. However, I think that this logic is flawed, and a scaling Life Tap actually causes some issues with warlocks.
Power Stats like spell damage and attack power work in a linear fashion for individual spells. They follow the general equation Total Damage = Base Damage + Coefficient * Power Stat. Pretty much all classes use this model to determine how the Power Stat affects the damage done by spells or abilities. Even warlock spells use this equation.
However, unique to warlocks, spell damage also changes the ratio of spells being cast. Consider a warlock who just casts Shadow Bolts and Life Taps. At zero spell damage, the warlock can cast 1.3 Shadow Bolts for every Life Tap. For about every 500 extra spell damage, the warlock can cast another Shadow Bolt before needing to Life Tap. 500 spell damage = 2.3 Shadow Bolts. 1000 spell damage = 3.3 Shadow Bolts. 1500 spell damage = 4.3 Shadow Bolts.
What this means that as a warlock accumulates spell damage, the percentage of time she spends on DPS increases. This is in addition to the fact that the individual spell's damage increases as well. So spell damage essentially has a double effect--a non-linear effect--on a warlock's DPS. It increases the damage of Shadow Bolt and, through Life Tap, increases the percentage of time Shadow Bolt is cast.
This shows that warlocks scale in a different manner than the spells of most other classes, and this probably makes them hard to balance accurately for all tiers of endgame.
On the other hand, if Life Tap didn't scale, you can replicate much of the same effect by downranking Shadow Bolt. A downranked Shadow Bolt has a lower cost, meaning it can be cast for a greater percentage of the time, and potentially doing more total damage than the higher ranked spell at some point.
So now I'm not really sure what we can conclude from this exercise, other than once again we show that being able to downrank spells causes problems in WoW. In many ways, costs are most important part of game balance. Being able to change costs without repercussion has caused a lot of balance problems in WoW. One of these days, I hope Blizzard takes this to heart, and removes the ability to cast downranked spells.
DPS warriors also have some of the same double effect, in that AP allows them to hit harder, therefore they generate more rage, which allows them to use specials more often. However, DPS warriors tend to hit the point where they have always have enough rage for their ability cycles relatively early in their progression, so it ends up not mattering as much.