What will they be charged with?
They can't be charged with making hoax bomb threats because they never made a bomb threat. The signs do not resemble bombs, nor were they intended to be seen as bombs.
They can't be charged with trespassing because the signs were all placed in public places.
The most I can think of is littering- one fine for each sign placed. Then again, the DA would have to demonstrate that people and businesses that routinely plaster telephone poles and bus stops with signs and fliers are consistently fined.
The union cops in Boston simply had nothing better to do yesterday.
Agreed