Have you read the book? It is clearly not his focus (or area of expertise!) but the book makes its premise clear: extreme inequality is inconsistent with the meritocratic ideals upon which modern capitalist democracies are founded, and can lead to social unrest. This is close to a direct quotation from the book's introduction --0 I would quote it exactly if I had the book in front of me right now. Maybe you don't agree with this premise, but I don't think you can fault Piketty for taking it as an assumption (explicitly!), particularly given that he is not a sociologist or political philosopher.

And in either case, it strikes me a significant contribution in itself to move the debate from "does capitalism and rising productivity raise all boats?" (answer: no) to "ok then...is extreme inequality really such a bad thing?"