I think the freedom from bras was as much a symbol as anything else. I remember my mother snarling as she shoe-horned herself into a girdle (a much more oppressive garment than a bra). And HER mother grew up wearing corsets (a garment that could be included in a regimen of torture).

Like men wearing kilts, it's an issue of freedom and choice (like the choice of whether or not to go regimental)