As quoted from http://www.thefreedictionary.com:

***Traditionally, hey was just an exclamation. Sometimes it expressed delight, sometimes a warning. Nowadays we find it used for emphasis as well, especially in the expression but hey. It is also a greeting. It is a short, colloquial version of How are you? and thus close kin to the informal salutation hi, which it seems to be replacing in many situations. Until recently, this greeting had a distinctly Southern flavor. The national survey conducted in the 1960s by the Dictionary of American Regional English found hey as a greeting restricted chiefly to Arkansas, Louisiana, Mississippi, Florida, Georgia, and the Carolinas. The friendly hey has since spread throughout the United States.***

So, 1) Yet another salutation formed from what was originally a warning.
2) Yet another Southernism shared by all. Hey, I like that! :)