A wireless end-user device has wireless wide-area network (WWAN) and wireless local-area network (WLAN) modems. One or more processors classify when an application is interacting in the device foreground with a user. For a time period when data for Internet service activities is communicated through the WWAN modem, the processors use a differential traffic control policy to selectively block and allow network access for an application based on whether the application is classified as interacting in the device foreground with a user. The processors provide a network access condition to the application through an application program interface (API), and thus can indicate that Internet data service is unavailable to that particular application based on the classification. A different policy may apply to WLAN modem usage and/or to different WWAN networks.