Why is there an isur me'ilah on a korban that is posul?

That's a very good question, Mendel. That is, the Isur Me'ilah would seem to be related to theft from Hekdesh. When a Korban is Pasul and is not worth anything to Hekdesh, why should there be a Chiyuv Me'ilah?

TOSFOS (2a DH Chutz) tells us that there is Me'ilah in the cases of Kodshim Pesulim mentioned in the Mishnah because "Im Alu Lo Yerdu," i.e. they are left to burn on the Mizbe'ach if they are put there (accidentally). Since they may be "offered" post facto, they are considered to be of value to Hekdesh. This indeed is the implication of the Gemara (2b, "Ela l'Rabba Kashye...), and this may be the intent of the Gemara on 2a (Kivan d'Re'uyim l'Kodshim Kalim...) as well. RASHI (Kerisus 23a DH b'Gasa) gives such a reasoning as well.

Be well and l'Hitra'ot!

-Mordecai