They are concerned in part because of the limited ability of Apple product users to opt out of revisions in its terms and conditions that include agreeing to the sharing.
They are the same legislators who sought information from Google on its collection of geographic data from unsecured WiFi nets. Google has said that collection was inadvertent.
In a letter to Apple CEO Steve Jobs, they asked for answers to a number of questions, including what products are being used to collect data?; when Apple started collecting it?; how many people it affects?; and whether it is sharing that data with carrier AT&T?