What is inappropriate dress in a flight

Saw this in news
http://edition.cnn.com/travel/article/thomas-cook-airlines-female-passenger-cover-up-intl-scli/index.html

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pennynoodles · 56-60, F

It didn't look offensive and certainly not provocative, especially as she is pretty much flat chested. However, I suppose there has to be a rule in place to stop people sitting there in swimwear.