First, using SaveAs isn't recommended. Print will give you better and more predicable results.
But this really isn't a Frame problem -- this depends quite a bit on your Acrobat set up because that IS your printer. And different versions (printers) have different capabilities. But you don't mention what version(s) you're using...
For example, in Acrobat 9, there's a good writeup on converting color spaces, which is what you're doing. But it happens in Acrobat, not in Frame. http://help.adobe.com/en_US/Acrobat/9.0/3D/WS06CB9258-2BBB-4526-B079-FB1840BAAF9D.w.html
Also in Acrobat 9 Extended, you can use the Preflight tools to force the conversion if the PDF already exists. On the Profiles tab, select Digital Printing, then the Digital Printing (B&W) to "Convert all colors - including spot colors - to grayscale."
Art