13 Oz restriction on mail?

Can anyone explain to me the new signs on the US Post Office boxes that state that due to heightened security restrictions, packages over 13 Oz cannot be sent from the mailbox, and must be shipped from a branch office. What is a 13 Oz limit going to do anyway? Have they determined that 14 Oz packages are historically more problematic than 13 Oz or less packages? Who picked 13 Oz anyway? Strange.