Someone in DefaultTrust is like a moderator responsible for cultivating a good trust network. They need to make sure that anyone trusting them sees many accurate ratings and no inaccurate ratings. Actual trustworthiness is important, but I can quickly respond to any abuses, so it isn't the most important factor in my decision to add someone to DefaultTrust.

The default trust network needs to work in this (somewhat counter-intuitive) centralized way, but you don't need to use the trust system in this way. You can use it more like a web of trust, adding only people who you trust to your trust list.