When a label can potentially overlap into other regions, Spotfire adds that white background for the text.
Although there is no easy way to get rid of the white background - there are two workarounds
1. Put labels on the outside of pie.
2. Increase the threshold so that only bigger pie sections are labeled (The labels for 5.6% section are the reason these white borders are appearing, if you increase the threshold to 6% the white border will disapper but the 5.6% label will not be shown as well)