There is a Biblical prohibition against tearing an item apart – Koreiah. This was done in the Mishkan when curtains would get holes by insects or wear and tear, they would rip the material a little bit larger than the hole so they could sew it up again. This being the prototype of Koreiah it only includes tearing for a constructive purpose. However, our Sages included tearing even for a destructive purpose. Since tearing destructively is Rabbinic there are certain types of tearing that were not included in their decree.