[ithkuil]

I believe it's more likely that the reason is that scarce resources have a higher value/price.

The society regulating itself (read: taxi unions and politicians) recognizes that if the number of taxis weren't limited, the prices of the fare would go down, usually favouring people which are desperate enough to earn some money (read either young or immigrant). This would drastically reduce the wage of the non-immigrant french/american/italian citizen who has to provide for it's own family.

If the regulation would impose a minimum fare (in order to disallow competition among taxi drivers), there are still concerns that individual taxi drivers would have a bad time competing with taxi companies (with drivers that don't own the car or license).