Actually it is considered both a poison and a drug. The problem is, some suppliers where (and still are) selling ecigs as a safer alternative. (Smoking Everywhere for example) Although I'm sure we can all agree that there is enough information out there to prove that ecigs are safer than traditional cigarettes, there have been no official studies done on the ecigs itself here in the United States. So legally they cannot sell it as a safer alliterative. It would take about 5 years in a few billion dollars to make that happen.
If they do sell it has a safer alternative, guess what? FDA waves their big wang and say "uh uh, no bueno" and pulls the product.