I wouldn't want to print it if I was in a small town, just because I wouldn't want to make Enemies. I still probably would... But that issue aside...
I also don't think it's Slander, or immature, or any of that stuff. Thats like saying posting a bad review on a website is bad form, and you should handle all issues one on one.
In the real world... that doesn't work. for all we know this guy had some work done and it turned out like Crap. He went to try and get it fixed, but the owner didn't give a crap. So this guy could be stuck... Maybe he's holding a grudge, maybe he wants a sign infront of his house with a hole in the roof saying hey, look what xyz company did to repair it, take your business elsewhere if you want a livable house.
Same with Protesting signs... This is protesting a business, should we as sign shops refuse to print signs for Ikea workers who are protesting crap wages? Or any union or striker because we may anger the Parent company? If there was a conflict of interest, I can see turning the work down. But If you turn down regular Joe schmoe Because he wants to make an Apple sucks sign, you're effectively preventing the little guy from Protesting and giving Apple an advantage.
It's a Free Country, so long as it's not Illegal or Immoral, I'd have no problem with it.