I'm sure there's several ways to look at it, but here's a few that came to mind.

First, like Mike, Saul doesn't want to let anyone in and open up a wound he's trying to cover up (Chuck and all that baggage). This why he politely recoils at the idea of visiting a shrink, and he accepts the job just get Kim to leave him alone.

Plus, in a way, the Slippin' Jimmy/Saul persona is all about going off the grid and the idea of not working for/giving into "the man," so whether he's conscious of it or not, Jimmy is signalling his relapse/moral decline when he writes that sign.

Plus, a message like that would also have a certain resonance in the early 2000s when America was still in the midst of Bush-era, post-9/11 surveillance controversies.