Matt,

 This is a feature, not a limitation of Surfice (and MRIcroGL). You can save bitmaps as either 32-bit RGBA where the background is transparent or as opaque 24-bit RGB with a fixed background color. The behavior is influenced by the Preference "Background transparent in bitmaps". 

Due to partial-volume effects, the pixels at the edge of the brain will be a mixture of background and surface colors. This means that for opaque bitmaps the edge pixel color is a mixture of background color and surface color. This will be an issue if you use a tool like photoshop to superimpose images or change their background color. On the other hand, while transparent images require more disk space, they will nicely feather the edge color with any other colors.