Why add salt to the water when cooking rice?
For rice it depends.  Cooking any starch in water will first cause the starch granules to swell and eventually tangle up with each other (the gelatinization).  Dissolving sugars or salts in the water slows down the process by raising the temperature the swelling starts.  While few prefer pasta as a stuck blob of strands, the same is not the case for rice.  I like my Basmati loose, but my risotto and sushi sticky, so salt may be required for Basmati and optional for Arborio.

There are many techniques for controlling the starch for rice.  To control the starch gelatinization of rice:


cook it like pasta with lots of water, then drain; or 
par cook it


Method 1 won't avoid the grain surface starch gelatinization, but it will help with stickiness (you may oil coat it after draining).  Method 2 delays the starch release allowing you to finish a risotto in seven minutes.  Cool for parties or for restaurants.