To be fair, obscenity laws don't seem (to me) to be in keeping with the rest of the laws surrounding freedom of speech in the US. They are shockingly vague and have been used to prosecute people for artwork (!!!) in the past. Court verdicts for such laws seem to be mixed - sometimes they go along with them, sometimes they side with freedom of expression.
> offers to engage in illegal transactions
Just to clarify, that means "offers to engage in crime". Other than the aforementioned obscenity laws freedom of expression is generally quite well protected in the US so there won't be a crime in the first place (and thus related transactions won't be illegal).
> If it's illegal to posses something, the government can ban offering to sell and distribute it.
I think you misunderstand slightly. If it's illegal to possess something then it is _already_ (to the best of my knowledge) illegal to offer to sell or distribute it. The question is what the government is and isn't allowed to ban possession of.
Interestingly, in the case of "obscene" materials possession itself isn't banned. Only import, sale, and distribution.
The issue with "People who create and distribute these stickers knowing their purpose should go to prison." is that such a wording seemingly bans them outright regardless of intent. That is an affront to freedom of expression. If I want to craft adversarial examples I shouldn't need to justify my intentions and seek permission up front. An action should need to be justified as illegal on a case by case basis, not the other way around.
Note that it is already illegal to deface street signs so by extension if you create and distribute stickers with the express intention that they be used that way then presumably you are already violating the law today. On the other hand, such stickers are not (currently or ever, I hope) inherently illegal in and of themselves.