They decided that it would be a good idea to start centralized, then over time wane themselves out of the "validation" of transactions until it's in the hands of a diverse group of validator nodes where no one person has the ability to change things.
Ripple labs has a very strict way of handling the XRP they hold. It's not just "lose structures" in place, they are using the Ripple blockchain to escrow 55 billion of the XRP and release it on a set schedule of (I believe) 1 billion per month, and any that is left over at the end of the month is re-escrowed. So they can't just "flood the market" at any time, there is a set limit to how much they can even sell without hardforking the blockchain in their favor.
The founders have free reign on their 20 billion XRP, but I don't see why they would ever use it to crash the price.
Profit motive?