The Gemara says The Parsha of korbon for Avodah Zara is written next to the Parsha of a regular Korbon Chatos. We learn from there that the Aveiros that you are Chayiv a Chatos are similar to Avoda Zara. Just like Avoda Zara has a Chiyuv Karies and a Chiyuv Chatos, so too all Aveiros that have a Chiyuv Kareis is Chayiv a Chatos.
Tosfos asks: once we're comparing it to Avodah Zara, why not say just as Avodah Zara one is Chayiv Skila (to be stoned) so too all Chatos only apply to Aveiros that have a Chiyuv Skila?
Tosfos answers: we only compare the Chiyuv Chatos to what is written explicitly in that Parsha of Korbon on Avodah Zara. Since there the Torah only writes Kareis therefore we can only use that aspect to compare to the other Aveiros.
A proof to that is from the Gemara in Makos that says that if someone transgresses by not bringing a Korbon Pesach or not having a Milah he doesn't bring a Korbon. The Gemara doesn't learn it from comparing it to Avodah Zara that there is a Lav,so therefore all Chiyuv Chatos need to have a Lav. We must say the reason the Gemara doesn't say it that way because the Pasuk by Korbon doesn't talk about the Lav of Avodah Zara.
(However, the Pasuk does refer to doing Avodah Zara, therefore the Gemara learns that one needs to trangress an Aveira in an active fashion. This excludes Pesach and Milah where one transgresses in a passive fashion.)
