Personally, I think they're pretty tacky- and so do a lot of people I know, but no one I've asked knows exactly why they've been painted.

"The project is called Garden in Transit and it's a mobile public art project to help "celebrate the 100th anniversary of New York's first traditional metered taxicab." The decals that decorate the cabs were painted by 23,000 children and adults from organizations around New York City. The waterproof decals are decorating thousands of cabs in the city."