What is a bama shayach here? It's completely different. If learning from a bama is legitimate, can we not extend this to zarim doing avodos, since they are permitted b'bama?
Tuvya Marcus, Jerusalem
1) The Gemara does ask a similar question to yours: How can one learn about something not done in its kosher way from something done in its kosher way? Rashi explains that the question is how can one learn from Yotzei concerning Bamah? Every Bamah is by definition "Yotzei" because there are no walls in a Bamah, so the correct way of bringing a Korban on a Bamah is with Yotzei. The Beis ha'Mikdash is very different because it has walls!
2) The Gemara answers that in fact we are not learning from Bamah. Rather, we are learning from the verse (Vayikra 6:2), "This is the Torah of the Olah." Rashi explains that there is "one Torah" (one law) for everything that goes up on the Mizbe'ach -- [and that law is] that one does not take it down.
3) The Gemara below (68b) also asks a similar question to yours concerning Zarim: We should learn from Bamah! Rashi (Kisvei Yad) explains that the question is that we should learn from Bamah that if a Zar does the Melikah which is in place of slaughtering for a bird offering, this should be Kasher b'Di'eved also in the Beis ha'Mikdash, and if the Korban has been put on the Mizbe'ach it should not come down. The Gemara answers that we do not learn from Bamah. Rashi writes that a Bamah is profane in comparison to the Beis ha'Mikdash, so one cannot learn Dinim for the Beis ha'Mikdash from Bamah. The Gemara challenges this by arguing that we do in fact learn from Bamah that if Yotzei was put on the Mizbe'ach in the Beis ha'Mikdash, one does not take it down! The Gemara gives the same answer that it gave on 51a: we are not really learning from Bamah, but in fact we are learning from "This is the Torah of the Olah."
(See also Tosfos to 84b, DH u'Motzei, in the Hagahah.)
Kol Tuv,
Dovid Bloom