TeleAtlas US street labels are turned on by default when a map contains a sufficiently small extent. Instructions provided are to increase the range of the default, so the street labels appear while the map is at a larger extent.
In MapImageOptions use the object displayLayers to turn on the appropriate street labels layer:
-- For the ArcWeb:TA.Streets.US or ArcWeb:TA.Streets.NA data sources, use the layer 'U.S. Street Labels (alternative)'.