Originally Posted by
ErrandGirl
I am a Canadian citizen, born & bred.
...
I'm still freaking out about my fingerprints... was it legal? Will they do it again? Can I refuse next time?
Apparently yes, it is 'legal'.
As per this page:
http://www.dhs.gov/files/programs/editorial_0695.shtm
Note: Customs and Border Protection Officers retain the discretion to refer a visitor for US-VISIT processing as part of the inspections process if there is a concern about the nature of travel.
so it looks like you were subject to discretion. Pretty much confirms that CBP can do any damn thing they want, but we knew that.