Hey, dumbfuck, most cabs in cities throughout the U.S. are not modified to exclude front seat passengers, like they are in NYC, for example. It is very common in most areas, particularly in areas not plagued by high crime rates, for passengers to ride in front. I've done it myself more times than I can count.
OP's story may or may not be true, but your Sherlock Holmesian sleuthing does absolutely nothing to diminish the credibility of the story.