Your question can also be asked on the Gemara in Makos (2b), which says that Edim Zomemin is considered a "Lav she'Ein Bo Ma'aseh" such that the transgressor does not receive Malkus. Indeed, the NETZIV asks this question in Bava Kama (4b). Tosfos in Shevu'os (21a, DH Chutz) also points out to the apparent contradiction between the Sugyos in this regard, but he does not explain why they in fact do not contradict each other (see there, and see ARUCH LA'NER in Makos 4b).

It appears that the intention of the Gemara in Bava Kama is that the witnesses caused a Ma'aseh to be done as a result of their testimony (that Ma'aseh being the G'mar Din of Beis Din), as Tosfos writes in Sanhedrin (65a, DH Ho'il), that it is considered more of a Ma'aseh since it is because of them that the defendant was found guilty. therefore, it is fitting to be mentioned among the list of Avos Nezikin (this is in contrast to a Moser). It was necessary to base this on the verse for "La'asos" in order to show that when the defendant becomes Chayav due to the witnesses' testimony, it is considered to be a direct result of their testimony, and it is not comparable to damage (Hezek) caused by a Moser, for such damage does not come about so much as a direct result of his actual words, but rather through the fact that a Nochri heard his accusation and acted upon it.

Nevertheless, the witnesses themselves certainly did no Ma'aseh, and therefore they cannot receive Malkus nor bring a Korban Chatas, as the Gemara learns here that the Aveirah of Edim Zomemin has no Ma'aseh (because they are "Yeshnam b'Ra'ayah").

I found that the Megilas Setarim of Rav Nisan Ga'on (cited by the Margoli'os ha'Yam in Sanhedrin 65b, #8) has -- in Kerisus and Sanhedrin -- the Girsa that "Edim Zomemim is different because Yeshnam b'Im ." One explanation of this is that even though the speech of Edim Zomemin is considered like a Ma'aseh (as the verse writes, "Zamam La'asos," as in Bava Kama 5a), nevertheless since the verse writes "v'Hayah Im Bin Hakos" in order to be Mechayev him Malkus, this implies that the Torah does not consider this a Ma'aseh to become Chayav for it Malkus and a Korban. According to his words, it is understandable that the Torah calls it a Ma'aseh and nevertheless it is excluded (by the verse "Im") from Malkus and Korban.

M. Kornfeld