We can answer as follows: The Rambam writes [Eidus 20-4]:
"ואין לעדים זוממין שגגה לפי שאין בהם מעשה, לפיכך אין צריכין התראה כמו שביארנו".
There is no concept of accidental or unwitting sin [שוגג] regarding עדים זוממין because their sin doesn't involve an action, therefore they don't require warning [which is usually used to distinguish between שוגג and מזיד].
The Raavad comments:
"אמר אברהם, זה הטעם לא ידעתי מהו. ואם מפני שאמר הכתוב תורה אחת יהיה לכם לעושה בשגגה, א"כ מגדף לדעת החכמים [דסברי שהיינו מברך את השם] לא יהא צריך התראה".
The Raavad says that he doesn't understand the rationale offered by the Rambam. If it is because the Torah says "לעושה בשגגה" - the rules of שוגג only apply to "עושה" - actions, then when one blasphemes Hashem [ח"ו], he shouldn't require התראה.
But he does!!
Dum de dum dum. Duuuuuuummmmmmmmm!
It would appear that we can say to resolve the opinion of the Rambam, that he doesn't mean as we would understand simply that since the עבירה isn't committed with an action, there is no exemption for שוגג. The Raavad correctly noted that there is no such distinction and even when the עבירה doesn't involve a מעשה, one who is שוגג is פטור, as we see regarding the מגדף who is only punished when he is מזיד. So when the Rambam said that a שוגג is חייב with regard to the דין of עדים זוממין he meant something else, as we will explain.
Rav Alter Shmuelevitz explained that the scenario of שוגג with regard to עדים זוממין MUST be a case of אומר מותר - the עדים זוממין thought that it is permitted to give false testimony. It is not reasonable to assume that the שוגג is that they imagined seeing something they really didn't see. We assume that people don't imagine seeing things that they really didn't. It makes more sense to say that we are talking about אומר מותר. According to this, when the Rambam said that there is no exemption of שוגג for עדים זוממין, he meant that אומר מותר isn't פטור.
It would seem that the root of the reason for this is that the punishment of עדים זוממין doesn't come as a punishment for sin like other punishments in the Torah but rather it is המשכת החיוב - a drawing of the punishment from the person it was intended for [through the false testimony] on to the witnesses themselves. THAT is the חידוש of the pasuk ועשיתם לו כאשר זמם לעשות לאחיו - the punishment that they wanted to give goes right back to them. Hence even אומר מותר gets punished because the Torah only exempted an אומר מותר from PUNISHMENT but not from false testimony which is defined as המשכת החיוב and not a standard punishment. Since it is not a punishment but a drawing of the obligation that they wanted to create back on themselves, the שוגג and מזיד here depend not on whether there was intention to commit the sin but on intention to testify falsely (and even אומר מותר had such intention). Similarly the Rambam writes (Trumos 5-8) about someone who took Truma on טמא fruits for טהור fruits, that if he knew that the Truma was טמאה but mistakenly thought that it is permitted to do so, he has a דין מזיד. So is explicit in Rashi Gittin 54a ד"ה בשוגג.
That is what the gemara means at the beginning of Makkos 2b:
מה הסוקל אינו נסקל הבא לסקול ולא סקל אינו דין שלא יסקל
If one who stones another, i.e., who testified that another is liable to be executed via the death penalty of stoning and was rendered a conspiring witness after that person was executed, is not stoned, as the halacha is that conspiring witnesses receive the punishment that they conspired to have inflicted and not the punishment that they actually had inflicted, then with regard to a conspiring witness who came to stone another and was unsuccessful and did not stone him, as he was rendered a conspiring witness before that person was executed, isn’t it logical that he should not be stoned?
That elucidation was based on Rashi. But Rabbeinu Tam learns differently:
לכך פירש ר"ת דה"ק, ‘ומה הסוקל אינו נסקל ... ‘ פירוש, אדם שסוקל חבירו באבנים ומת, דנדון בסייף ולא בסקילה, הבא ליסקל ולא נסקל - עדים שמעידין איש פלוני שמחויב סקילה ולא נסקל על ידם, אינו דין שלא יסקלנו?
Rabeinu Tam's explanation: The 'Kal-va'Chomer' is from a case where Reuven stones Shimon to death, yet he himself is (sentenced to death by the sword, but) not stoned, how much more so will he not be stoned for merely intending to stone him.
From here the gemara proves that the דין of כאשר זמם does not depend on a case of what would happen had he done the act himself. It would seem that the gemara means that if the nature of the חיוב was a PUNISHMENT for their testimony, then it makes sense that we can't give them a more severe punishment than if they had actually carried out the דין and given סקילה. Then the punishment would be not סקילה but סייף. The fact that they don't get סייף but סקילה [for testifying that someone performed an act for which he is חייב סקילה] is a "game changer". It teaches us that the obligation of the עדים זוממין mirrors the very same חיוב they attempted to inflict upon the accused.
[עפ"י תורת הגאון רבי חיים שמואלביץ זצ"ל]
