Stellar is not currently decentralized. The use of a single validating node is a temporary measure while they rewrite their consensus algorithm. This is obviously far from ideal, but the Stellar foundation has been transparent that this is a temporary measure. In the original design, and in their stated mission that they've never wavered from, this will be a open protocol where anyone can join as a validating node. But they needed a stop gap measure while they fixed this ledger fork issue. Again, far from ideal, but let's not jump to harsh conclusions either.