It shouldn't. The mandate was forced on the industry despite serious warnings about the problems MTBE and other oxygenates could cause and doubts about the need for oxygenates in the first place. MTBE is soluable in water precisely because it has an ether group which contains oxygen. Most organic compounds are non-polar and are not very soluable in water. The fact that most organic contaminents are both insoluable in water and less dense than wather (hence they float) that allows them to be rather more easily separated from water. It is the soluability in water and the ability of MTBE to actually migrate through aquifers faster than water that causes the contamination problem.
You are totally wrong. See Post 26 and the reference link.