Why does tap water enema cause water intoxication?

Answer:

The intestines can absorb water.

A tap water enema may cause electrolyte disturbances similar to when a person drinks more water than their body can metabolize. As a precaution it is best to add a teaspoon of salt to each 1000ml of water to prevent absorption and keep the sodium balance.

I have included several links that discuss water intoxication as well as water enemas.
Here are some more details and a link to an informative article:
When sodium levels drop in the fluids outside the cells, water seeps into the cells in an attempt to balance the concentration of salt outside the cells. The cells then swell because of the excess water. In severe cases, cerebral edema and fluid in the lungs manifests. All of which can be lethal, when left untreated.

As the water content of the blood increases, the body cannot excrete the fluids fast enough. The excess water then goes to the bowel, which pulls salt into it from the body diluting the concentration of sodium in the body tissues leading to massive cellular damage effecting muscle, brain, heart liver, and kidney function. When cell death occurs, water intoxication becomes water poisoning.

Among sodium's varied roles, it helps to maintain blood pressure, electrolyte equilibrium, and allows organs, muscles and nerves to work properly.
http://www.associatedcontent.com/article/146397/water_intoxication_how_much_water_is.html?singlepage=true&cat=5