That said, people's right to display such things is as protected under the 1st amendment as our right to own the guns is under the 2nd. I'm not going to attack them on it - I just wouldn't display one.
Well, that's an issue we run into a lot -- what is the 1st Amendment issue here?
The 1st Amendment says
"CONGRESS shall make no law..." It, like the other Amendments, enumerates that THE GOVERNMENT shall not try to stifle your right to speak.
It does NOT say that what you say, or print, or sell, is immune from criticism or that you are immune from suffering repercussions of your decisions to write or say or sell something.
Sure, they have every RIGHT to print those and sell them. We have every right (and perhaps a "duty" to ourselves, our community, and our interests) to discourage them from doing so.
(Especially, considering that they most probably simply got caught up in wanting to produce a bit of zombie chachka and never stepped back far enough to realize what their sign really says to 99% of observers!)
The 1st Amendment and their right to free speech is in no possible way infringed by letters, emails, phone calls -- or if it came to that, by boycotts, protests and picketing!
Should the government step in and prevent them from printing this? ABSOLUTELY NOT. That would be a 1st A. violation!
Should WE, the shooting public, and their potential customers step up and try to influence them not to do such a stupid and harmful thing? ABSOLUTELY!