Was there ever a stated reason for the giveaway?
Was this in response to the ridiculous criticisms they received such as "centralization" and concentration of wealth?
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The best description of XRP distribution/giveaways I've seen is on Ripple Labs XRP Distribution web page.
The whole page is worth reading, but I'll just include a few select quotes that may address the question:
Users don’t need to hold XRP if they prefer to transact in other currencies (e.g. dollar, euro, yuan, gold).
The inventors of the Ripple protocol created 100 billion XRP at its inception, and no more can ever be created according to the protocol’s rules. The creators gifted Ripple Labs 80 billion of the 100 billion XRP in existence.
Ripple’s method of confirmation, called consensus, doesn’t need mining; therefore, it requires comparatively negligible computing power, confirmation time adapts to network latency, and transactions are immediately irreversible once confirmed.
Because Ripple’s new architecture does not require mining, the creators of Ripple had a choice: distribute XRP exclusively via mining or diversify the distribution methods.
Distributing value is a powerful way to incentivize certain behaviors. Bitcoin’s mechanism, for example, led to an explosion of processing power devoted to bitcoin mining. Our goal in distributing XRP is to incentivize actions that build trust, utility and liquidity in the network.
There is no distribution mechanism for the XRP in the Ripple Protocol, instead all XRP were created with the network's emergence. However, XRP are necessary to create accounts, create trust lines, and to pay for transactions, so every user needs some in order to use Ripple.
So, Ripple was bound to have all XRP afer creating the network. They could have started selling them, but since at that point nobody used Ripple, why would anybody pay for XRP at that point?
Consequentially, there would be no other way to spread the XRP besides giving them away. As far as I remember, giveaways were announced in the very first thing I read about Ripple. So, no, giveaways were not a response to criticism.