They all do it!!!!
I went to yahoo.com and searched for CSCO. The first result was from Yahoo Finance.
I then searched for "Edinburgh" and the first result was from uk.travel.yahoo.com.
In fact Bing was probably the best on this. They came up with Comstock, Google then Yahoo on the CSCO search, but I think Comstock was "hard coded" as another search for "csco finance" came up with yahoo then google. Microsoft clearly have a contract with comstock.
The point is that the search providers are trying to give users the information they want. To give the level of information they put on that first entry on the list, they have to choose one information provider as otherwise they would have to call multiple API's depending on the users preference and they would have no control on the speed of getting the data. By using their own servers to give the information they have control of the users experience.
As others have said, the choice is there as users can go to yahoo, bing or any other search provider. The only thing you could possibly accuse google of is not labelling the search result as being prioritised to give the user appropriate information.