In the US, there is no issue in trial courts about anybody "deciding" what the law is: not lawyers, judges, or jurors. It is already decided what the law is. Jurors decide whether the evidence and the judge's statement of the law logically lead to a verdict of guilty, or not guilty. The judge will (in most states) read instructions approved by a judicial committee, the contents of which are based on case law – we could say that that committee "decides" what the law is. The judge must make decisions (based on case law) as to which instructions to read, and "errors" in choice of instruction are a basis for appeal.
The problem with a poster containing "the text of the law" is that the law, in the US, is more than just statutory text, there is also a huge surrounding corpus of legal precedent which tells the legal profession how to interpret the underlying statute. The courts are supposed to educate jurors as to what the law is in a specific, uniform fashion, not just reading the statutory text (which is often impenetrable), but what it is deemed – by the courts – to mean. When a law refers to a "reasonable" belief, jurors will probably not get much help as to what "reasonable" means. In the context of the "reasonable doubt" instruction, in Washington, that is said to mean
A reasonable doubt is one for which a reason exists and may arise from
the evidence or lack of evidence. It is such a doubt as would exist in
the mind of a reasonable person after fully, fairly, and carefully
considering all of the evidence or lack of evidence
This is logically consistent with thinking "I don't like the DA": and it doesn't apply to defenses based on reasonable beliefs, where the individual juror has to judge what constitutes a reasonable belief. A printout of the pertinent section of the state or federal code won't help the juror to grasp what the law is.
An attempt to display a poster stating the law would be instantly squelched.