As for the reason why, it's because a freighter doesn't need to carry that much fuel and removing the pumps and plumbing saves weight. The HST doesn't get filled until the fuel load reaches about 150 tonnes. I can't remember the exact figure as it's 7 years since I've flown it. If you're getting anywhere near that figure, youré reducing the ZFT. On a freighter, it makes better economic sence to fly at max ZFW and tech stop rather than leave payload behind to increase range.
The HST is not a trim tank like it is on the A330/340. It's just another place to stuff fuel.