There seems to be an unwritten rule among billionaires: dont criticize each others lifestyle.

Bill Gates and Warren Buffett, for instance, spend a lot of time pressing other billionaires to give away more money and do more for society. But they have never ridiculed Paul Allens megayachts and private jets, or Larry Ellisons real-estate sprees.

Perhaps that is why Indian billionaire Ratan Tata (left) has sparked controversy by weighing in on the vertical palace owned by fellow Indian billionaire Mukesh Ambani.

In an interview in The Sunday Times of London, Tata was quoted as saying that the 27-story home, reportedly costing up to $1 billion, may have been ill-considered in a country of such widespread poverty.

"It makes me wonder why someone would do that..." the Times quoted him as saying in this article (subscription required). "The person who lives in there should be concerned about what he sees around him and [asking] can he make a difference. If he is not, then it's sad because this country needs people to allocate some of their enormous wealth to finding ways of mitigating the hardship that people have."