I am not going to post a bunch of links because it's boring stuff, but experiments show that a jury will not actually disregard testimony because of instructions from the judge. If it was illegally obtained, the information is pertinent to the jury anyway. If it is declared "unreliable," however, a jury will weigh it even more carefully than usual.
So the goal of a lawyer opposed to testimony is to keep it from being heard. Watchtower does this also. And members cooperate because they are not like jurors, but more like mindless followers of this rule. If Watchtower says they should shun, they will shun. Luckily, more and more are waking up, if only on rules like this, and not shunning like they are supposed to. Still, let the DF/DA'ed person start bashing Watchtower instead of just talking family matters and sports, the mindless follower programming will kick in.
I suppose beating this courtroom analogy to death will allow for us to recognize that JW's are not like jurors at all. They are "Witnesses" for Jehovah, meaning they are on the side of Watchtower automatically. So if their lawyer says they are to testify contrary to some other testimony and they are better off not hearing that contrary testimony, they automatically agree.