Just tossing this one out there,.. the pipeline operators wouldn`t be able to clean out the oil wax/parafin and sludge that builds up inside the line, with a "pig".  A "pig" is a big round 'scraping' tool "sent through" the big TAPS line through a special door and it scrapes the whole inside of the TAPS pipe all the way down as needed.  Any other lines inside would prevent that important maintenance as it wouldn`t fit, and sooner than later the sludge and crud would prevent safe operation of the line.