I think it's an anachronism. It's been embedded in the act from the 19th century and it's simply been carried forward. In some ways it's surprising that it was carried forward in the 1970 revision to the Statistics Act. That was the moment when I would have thought that the world had evolved and people would no longer think that jail was an appropriate penalty. Possibly it's simply that the penalty has never or rarely been applied so people just never focused on it.
I can't satisfactorily answer the question, but I really think it's something that's been simply carried forward thoughtlessly through time.