The ice cream industry does not fit the oligopoly model very well.
In an oligopoly, there are only a very few sellers of a given product. The sellers are all so big in terms of market share that the actions of any one seller can have an impact on the industry as a whole. This is not the case with ice cream.
With ice cream, practically every restaurant in America serves it. It is true that there are relatively few big chains, like Baskin-Robbins and Coldstone Creamery, devoted mainly to ice cream. However, ice cream is still available from many other sources. Therefore, the ice cream industry could hardly be called an oligopoly.