Whilst it's true that other planets contains water deposits, no where but earth contains it in such abundance and so accessible, as the article in io9 states.
As a rational human being that considers confrontation to be problematic, sure: I'd tap up one of the frozen planets for water underneath. But the TET was perhaps uncaring about the ramifications of ressistance on Earth (which, as the movie reveals, was obviously a fatal mistake).
Also, water only exists in its 'liquid state' on Earth, due to its Goldilocks status. It's probably much easier for the harvesting process to gather the substance in a maleable, liquid form than as huge blocks of Ice that need to be thawed out.
Also, as Oliver C has pointed out, it wasn't the water itself the TET was after, but the Deuterium contained within it. Deuterium is aparrently incredibly easy to extract from SEA WATER using centrifugal force.
A key advantage of deuterium is the fact it can be easily extracted from sea water without harming or polluting the remaining water in any way. It is estimated the world’s oceans contain enough deuterium to supply global needs for many, many generations to come.
So the TET was quite environmentally considerate, it seems! I'd imagine wiping out the worlds population of humans certainly put a stop to global warming. The Nihilists amongst us should be thanking it!