Because with a hand covering the breast, the model appears to be nude.

Let me try and put it another way.
If you show an image of a model covering her boob with her hand to 100 random people and ask them "Do you think this model is wearing clothes?", what do you think they will answer? (apart from "Who cares!" or "Can I have a copy please?!" and other things of that nature, heheh.)

They will say "Sure she's naked, I don't see any clothes..." or maybe "Is this a trick question?" But my point is, a vast majority of people will look at a photo/image where a model is holding/covering her boobs with her hands and say "That's nude."

Now some people may not like nipple pasties, they may think that they're crude or insufficient, and I'm surely not going to argue that you wouldn't wear them to work - but then you wouldn't wear your underwear (only!) to work either, would you, or a bikini (unless you're a lifeguard!) But I don't think anyone would argue that a bikini or a bra is not clothing. As long as the pasties fully cover the nipple and aureolae, Renderosity is happy to accept them as clothing and happy with their use in thumbnails.

[Edited for clarity]

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire