I understand that a phrase like piss off can cause upset, and why it might. What I don't understand is why anyone would be upset by its use on the suggested sign.
In my locality the girl street-light fairies used regularly to feel pissed off with the boy ones because the boy street-light fairies, according to my five year old source of this information, were a bit slapdash when extinguishing the street-lights and kept leaving some on, so the girls often had to check and finish the job as well as doing their own share.
My 5yo source of info probably did not use the expression pissed off about the girl fairies, but I assure you that that's what she meant. I know exactly how they felt. Flippin' half-a-job types piss me off as well ?