They didn't "collude" with the government. The whole premise of taxi medallions was to limit supply to maintain taxi driver wages and limit oversupply of cabs in a crowded city. Indeed, the number of cab medallions issued has outpaced population growth in NYC over the last several decades.
As for "Goldman Sachs-esque" returns, the price of a taxi medallion increased from about $100k in 1985 to $250k in 2005. Less than the return most Americans got on their house over that time. There was a major spike in the last decade, but it doesn't seem to be correlated with any real change in demand for cabs, or any specific regulatory action. As far as I can tell, the spike in price is just the result of the general attractiveness of safe but low-yield investments in this poor economy.
In any case, these are $1 million investments that net maybe $50-75k per year, and are appreciating quickly but not more so than say real estate in SF or NYC. The owners of these medallions aren't big money players. The investors behind Uber are.