There was a thread on twitter (
https://twitter.com/taoeffect/status/832041974735654912) debating this question and I wanted to move it here since discussions on twitter usually aren't that productive.
In my view Stellar is definitely permission-less. Anyone can run a validator. You don't need permission from anyone to join or use the network.
Greg, in the case that you are concerned with that there could be some "cartel" of validators that are all listening to each other and no one else, it is trivial for people to set up their own validators outside this cartel and listen to each other.
Keep in mind that this can also easily happen in bitcoin, if a cartel of miners with over 51% of the hashing decide to stop accepting blocks from people outside of cartel they can do this. The difference is in bitcoin it would be hard to ignore the cartel and there is economic incentive for the cartel to take all the blocks.