I agree with you and think you made a very important point at the end of your post. It is not easy for children to go back and forth with different standards in BMs and BFs homes.
Our situation is the same in that SDs have discipline at our house, and NONE at BMs. Every time we picked them up, it would take a few hours for them to adjust to our rules--sometimes even a whole day. Then they would revert back to BMs "rules" a few hours before we were supposed to get them back. Very frustrating for me!!
I am still working on being the adult, and having a bit more patience. We don't use physical punishments in our home, rather we use time outs and loss of privileges. But, looking back I think I was still a bit too harsh in some circumstances, and rationalized it by saying, "That's how I will treat my own children." My SDs have grown up in a home that is not mine, and have had experiences that my children will never have, and so I cannot treat them exactly the same. However, they will still be held to the same standards in our home. It's a very fine balancing act....
Another note on not spanking--we have not seen my SDs since July 2002 because BM accused my husband of abuse. As I mentioned earlier, we do not use any type of physical discipline in our home. But SDs were upset about discipline/chores/etc. in our home, they lied to BM and to counselor, and the judge believed the counselor. So just be careful..... They can turn the littlest things into something huge!