You then press the "find my friends" button and are surprised by the fact that it sends information about your friends to Foursquare?
The only valid complaint you could have is if they stored the information on their server permanently but based on this conversation they don't which is the best procedure.
The second you click "find my friends" you've opted into this. How the hell do you think they are finding out who your friends are? Magic?
There is a fine line between annoying the user and doing things without their permission and in this scenario Foursquare is better than the majority of other applications out there.