this idea seems like it would set a dangerous precedent. If people decide to start blocking coins doesn't that defeat the purpose of bitcoin?
Therefore you need to define your purpose of Bitcoin. My purpose is probably not your purpose.
Exactly! Your purpose isn't my purpose. So I'm not going to tell you how to use yours, and hope you don't try to legislate how I use mine.
Freedom isn't freedom if it's restricted. The only freedom that might should be restricted is the freedom to do harm to others. I agree that the FBI does harm to others, but in this situation (specifically selling seized bitcoin) they are not doing harm to anyone.
EDIT: I expounded on my argument.