I remember Richard (who seems to be a protector of the island) allowing them to leave? Surely this was not wise? Or was this a way of making the island harder to find for Widmore etc...?

I remember Richard being opposed to it, but Ben overruled him.

This is a glaring inconsistency which I have seen raised by a number of people and, in my opinion, never satisfactorily answered Both Ben and Richard just agree to let them go The only thing I can think of is that maybe their going and coming back was supposed to happen so that even if Ben and Richard tried to prevent it they would have failed...the universe would have course corrected? Ben and Richard knew this and just let them go?

Or maybe they knew the 06 weren't intended to go and so the island would bring them back?

That's the best I can do

I remember Richard being opposed to it, but Ben overruled him.

Richard made an agreement with Kate and Sayid to let them go if they help them save Ben.

Upon being freed, Ben acknowledged the agreement.

I believe the only people that they made a deal with and were 'allowed' to go were Michael, Walt, Kate an Sayid. And Locke seemed to be specific about it being 'Jacks' fault and he should not have left. However he can not return unless all go with him to recreate the original circumstances that brought them there in the first place. So when Richard said to Locke during one of the flashes that he must bring 'all' of his people back was because he knew just as Eloise did, that the original circumstances needed to be recreated and John would have to die to be Christians proxy.