Excluding specific cases where there are other factors (the law, religious reasons, etc.) I don't think that an airline should have the right to dictate what a customer wears, unless it's published in the Ts & Cs.

There are clearly things that are in poor taste but that's a matter of personal opinion and can't be forced onto others.

The woman in the photo is not breaking the law and isn't displaying what I would call an inappropriate portion of her breasts for the location she was traveling from/to.

If she was flying to Riyadh or Tehran, for example, it would possibly be a different matter.