This is one explanation given in part by "Grammar Traps".
"Lie" is, in this context, a verb meaning to recline. It does not take an object. Its principal parts are "lie," "lay," "lain," and "lying."
Examples: Every night I lie down. I lay down last night. I have lain down many times. I am lying down right now.
So, it is supposed to be used in a pas tense manner; now as to why we use it incorrectly. If you do something long enough, (and this will include other countries as well) it will become an accepted expression; Ain't is a fine example, once thought to be improper ( and still is), it made it's way into the dictionary.