They post those signs for the "listen to the birdy's chirping" people.
That sign makes them feel all important and validated about their choice to be "innocent victims of crime".
After the criminals get done with them,then WE will have to deal with them because criminals don't listen or read the "birdy's are chirping" signs.
That sign might as well say,
"INVITED! ALL CRIMINALS CARRYING GUNS!"
"WE ARE ALL DISARMED!"
"COME AND ROB US!"