its actually part of the straight edge culture. They put them on their hands to show the lack of a wristband i beleive...
The 'X'
At punk rock shows, it became common practice to mark an
X on the hands of under-aged concert goers to ensure that the
bouncers would recognize a minor attempting to drink alcohol. Some people interpret this as a symbol of
Ian MacKaye's
"don't smoke, don't drink, don't fuck" ethos. Some interpret the three
Xs as representing "Body", "Mind", and "Soul"—although three Xs have
also been used as an abbreviation for hardcore punk in general.
Many adopters of the "straight edge" lifestyle voluntarily marked
their hands in the same way to show their commitment to refusing
alcohol. Also widespread is the tattooing of the X symbol on other
parts of the body or wearing it on clothing, pins, et cetera. Three Xs (
XXX) have their origin in artwork created by
Minor Threat's drummer
Jeff Nelson in which he replaced the three stars in the band's hometown
Washington D.C. flag with Xs.
The X is considered both a mark of negation and a mark of identity.
Attaching the X to one's name or band name is common practice for
straight edgers. For example, 'John Smith' would become 'johnXsmith',
or 'xjohnx'. "Straight edge" is sometimes abbreviated
sXe (S.E. plus an X) following much the same logic and hardcore is sometimes abbreviated "hXc".