The problem with cabs is when they are circulating looking for work. They are causing congestion for no real benefit. Regulation caps how much useless congestion there is by capping the number of cabs on the road.

Limo companies don't cause this unneeded congestion because they only go out when they have a job to do, so there is no need to regulate the number of limos on the street. Whether or not Uber should be regulated depends on what their drivers do with their down time. Do they trawl the streets or do they get off of them? It seems like they should get off the road since they can't really pick people up without a reservation. However they may trawl good spots for pickups like near bars or something in which case they are causing unneeded traffic and should be regulated.