Originally Posted by barbell

As a data point, these signs do technically have the information you mention. Looking around for something else, I stumbled across the following links, which are the signs that I've seen, poorly placed, in the airports that I have transited.

MMW

http://www.tsa.gov/assets/pdf/mmw_info.pdf

Backscatter

http://www.tsa.gov/assets/pdf/mmw_info.pdf

The backscatter one does, indeed mention, in bold, that the scanner is optional. (n.b. I have not personally encountered the backscatter. Hence I have not seen the sign ORDOfcr describes.)

However, the poor design and placement of these signs makes it so that understanding that point is not a given for the majority of travelers who don't even know what they are stepping into.

Of course, they don't really describe the alternative as we've heard it relayed here and elsewhere. And that is also a problem.

Let me add my thanks to ORDOfcr for his/her professional and reasoned response. We'd get a lot further if everyone adopted this posting style.