Ahhh.. I finally found it. See section 7 of this PDF
from nyg2010.com.
It is intentionally vague.. basically, as I interpret this.. it is up to them (individual security employees) to make and change the rules at will.
the Licensor, the Giants or
other event sponsor, and that such Regulations may vary over
time or under changing circumstances. Licensee further
understands (i) that such Regulations, among other things, are
intended to promote the safety of all persons and property,
protect the Stadium, and enhance the pleasure of the game
day experience, and may therefore restrict or prohibit, without
limitation: unruly or disorderly conduct, the use of
intoxicating beverages, possession or use of illegal drugs or
controlled substances, sale or distribution of material while on
Stadium property, offensive or obscene language or displays,
damage to Stadium facilities or property, and possession in
the Stadium of items that in the reasonable judgment of
security personnel are or may be contrary to the purposes of
such Regulations, including without limitation, beverages,
cameras, food, explosives, paint, recording and transmitting
devices, signs and posters, sound devices, weapons or
umbrellas, and (ii) that patrons, their vehicles and belongings
may be subject to search to enforce such Regulations.