In Paris, the stations offer you an economic incentive. Unlike in Manhattan which is comparatively flat, riders in Paris enjoy coasting down from Sacré-Cœur and not so much peddling back up again.

So vélib encourages you to ride uphill:

> Return your bike to one of 100 stations perched over 60 meters above the rest of the city, and 15 minutes of free riding is added to your account. Riding uphill is now a power-up. The free minutes are a bonafide reward, too, saving money on destinations that are often outside of the free 30-minute radius. More bikes make it back up the hill. Since the program was launched in June, over nine years of bonus minutes have been awarded.

-- http://globalmoxie.com/blog/velib-bonus~print.shtml