They are legally required to allow an opt out. It is part of what allowed the scanners to be used in the EU.

The only place that does not allow an opt out of a body scanner at an airport check point is Australia (which to do so, the government backtracked on what they had promised, then ignored every single public comment on the matter). Even then, in the 3 years since they got forced on people departing Australia, I have yet to be forced into one.