Usually such limitations on testimony arise from the need to follow the rules of evidence. Hearsay, speculation, and opinions from someone who is not an expert are the three most common reasons for limiting testimony.
If you are aware of Fact A and Fact B, and you logically look at these facts to arrive at Speculation C, you won't be allowed to testify about anything except Fact A and Fact B. But rest assured that if Speculation C is the logical conclusion, then the jury will also arrive at Speculation C.
It's clear that you don't like these limitations. But if they weren't in effect, it would lead to far greater miscarriages of justice than what you have experienced.
Of course any attorney who instructs a witness to give false testimony should be disbarred.
Your reply only addresses a small part of the system. Both as a juror, as a witness for the defense in some cases, and as a witness for the prosecution in other cases, and as an impartial observer in court projects that monitored thousands of cases ( I took a night job specifically so I could monitor court cases daytime.) I have seen, and it has been widely documented by others that
ATTORNEYS LIE they encourage their witnesses to lie and more often, to withhold the truth. I have been directly instructed by attorneys on both sides to lie under oath. I have witnessed many cases where a friend waiting in the hall was advised to lie. I have seen many cases where a witness was not called to testify by the lawyer because the lawyer was afraid he would tell the truth... and the lawyer openly stated that was why he was not called. These cases have involved both lay and professional witnesses. I have seen cases where a police officer, caught in a lie that was metaphysically impossible, changed his story and told the truth, and the judge threatened him with contempt unless he persisted in the absurd lie. My observations have been mostly in Cook Co IL but also in 10+ other Illinois counties.
Police work within this framework of a corrupt system of corrupt lawyers.
THE COMMISSION REFUSED TO EVEN LOOK AT THE TRUTH. MEMBERS AND STAFF WERE SELECTED WITH A NARROW PRE-SET BIAS THAT FAVORED THE LAWYERS.
The parallel discussion on here about statistics is true and good. But it does not get to the fact that the system is broke.