The little I know: did you actually read the teshuvos from R’ Moshe?
http://hebrewbooks.org/pdfpager.aspx?req=921&st=&pgnum=315
Even the one you refer to clearly says it’s not assur, and for that reason, it’s not lifnei iver.
http://hebrewbooks.org/pdfpager.aspx?req=919&pgnum=67
The first one linked was written in ???”?. There’s a third one, written a few years later, about smoking on Yom Tov.
Whether he would have assered with more current information is arguable, but that he actually said that it’s not assur is not.