because it does not make sense to integrate uncalibrated files... WBPP is primarily a tool to match your lights with darks and flats. the integration it produces should only be used as a preview anyway.
rob
edit - well, more correctly WBPP is also a tool for evaluating sub quality and weighting them appropriately for integration, as well as light/dark/flat matching. anyway pixel rejection would be found in the integration tab of WBPP but again it is recommended to load the _c_r or _c_d_r files from WBPP into ImageIntegration separately to create the master light.