Answer
Beginning at version 10.1, the Bing Maps logo is contractually required to appear on all maps that consume Bing map services. The Bing logo draws on top of all layers to ensure that it is visible no matter what other layers are in the map. This causes all of the layers in the map to rasterize on export, which may additionally cause some features to appear slightly blurry in the output export file.
This is a known limitation when working with Bing map services at 10.1.