Why is it forbidden to cut toilet paper on a saturday?

One of the 39 Melachot, or forbidden categories of work on Shabbat, is "Mechatech", which is cutting something to a specific size or shape. Cutting toilet paper is usually assuming to be included. If one is in the uncomfortable position to find that one needs the facilities and there is no uncut paper- he should try to tear the paper without caring exactly where it tears- and try to tear it in an atypical way- such as, by doing the tearing with one’s teeth- This is considered to a prohibited activity in an unusual way, which is only prohibited because of rabbinic injunction, and is permitted when human dignity is at stake.