Since when? Show me the law.
Pain to find, but seems to be based on -
720 ILCS 5/21-6 -
(a) Whoever possesses or stores any weapon enumerated in Section 33A-1 in any building or on land supported in whole or in part with public funds or in any building on such land without prior written permission from the chief security officer for such land or building commits a Class A misdemeanor.
Sec. 33A-1. Legislative intent and definitions. -
(2) A Category I weapon is a handgun, sawed-off shotgun, sawed-off rifle, any other firearm small enough to be concealed upon the person, semiautomatic firearm, or machine gun. A Category II weapon is any other rifle, shotgun, spring gun, other firearm, stun gun or taser as defined in paragraph (a) of Section 24-1 of this Code,
knife with a blade of at least 3 inches in length, dagger, dirk, switchblade knife, stiletto, axe, hatchet, or other deadly or dangerous weapon or instrument of like character. As used in this subsection (b) "semiautomatic firearm" means a repeating firearm that utilizes a portion of the energy of a firing cartridge to extract the fired cartridge case and chamber the next round and that requires a separate pull of the trigger to fire each cartridge.
State references -
720 ILCS 5/21-6 -
https://www.ilga.gov/legislation/ilcs/fulltext.asp?DocName=072000050K21-6 .
Sec 33A - 1 (Down at the bottom are the definitions) -
https://www.ilga.gov/legislation/ilcs/fulltext.asp?DocName=072000050K33A-1 .
https://www.akti.org/state-knife-laws/illinois/ makes mention that the state need to prove you knowingly broke that part of the law.