5b Ein Mazhirin min hadin

5b Ein Mazhirin min hadin from Achoso

I was bothered for many years how we learn out ein mazhirin min hadin from ervah of your sister, because that is a chiyuv kares. Then the gemara learns out to ein mazhirin in a makos case from gzeirah shava rasha rasha, but that is equating makos and misas beis din. Why is there no direct limud from misas beis din, and why is kares seemingly equivalent to misas beis din?

For many years I had no answer. Yesterday I saw Rashi, Megilla 7b s.v. af. saying that kareis is like misas beis din. He doesn't explain why. It could be because if someone violating a kareis prohibition was warned for malkus, he's patur from kares, so it is like "misa" from beis din, or because "malkos bemakom misa omedes." Or because of kipa - Sanhedrin 81b - Beis Din can potentially kill someone who's chayav kareis several times.