We saw from the treatment Glory dished out to Spike that anyone who could be expected to know the answer could be tortured and might give out the information. And all of Buffy's close friends could be assumed to know who the key was. The continued torture almost happened when Glory ripped out the wall of the building and was not carried out only because Tara in her altered state gave Glory the information, so no further torture was needed. Otherwise they would have been picked off one by one until someone said something. It is necessary to have secrets sometimes and this is one of those times. Certainly Riley would know about confidentiality in his capacity as a part of a military organization. With the choice between:
"Boo hoo, she didn't tell me about this."
and
"The world could be sucked into a mingling of all dimensions and transformed into hell."
if Riley didn't get that the second idea is far more important, then he isn't much of a soldier or much of a man. Don't forget, Riley had been devoted to an organization that has some ideas about demons and the occult world that are very different from what Buffy knows (and Buffy's information is correct). Maybe Riley would think that the might of the US military could take out a God, and would fail in the process. Or maybe he would have the right idea but would not be able to convince the chain of command.
I think Buffy made the right choice in this particular instance. Given the choice to be reinstated in the military after he left with all its arbitrariness and fixation on who has the right to take action or leave the group that almost killed him, he chose to revert to what he had been.
I see your point, and I agree to a certain extent (mainly on the fact that Riley going back to the Initiative is pretty, well, reckless and stupid).
But, Buffy says she's not telling the Scoobies to protect them, and not really to protect Dawn (of course, it's part of the plan).
Glory would have tortured anyone and everyone until she found someone who knew about the key. Now that you mention this, I wonder why she didn't start by that, it would have saved her a lot of time.
So if Buffy didn't tell them to protect Dawn, thinking that if one of her friend was kidnapped and tortured, he couldn't spill that Dawn's the key, I really do see why she didn't.
But not telling them to protect them? They are constantly in danger, and she even states to the Council how brave they are. For me, it's because she doesn't trust them enough.
I am not mad at Buffy or anythig (well maybe for lying to Riley, but she has trust issues and he really didn't handle it well), I just do not understand why after 4 years and a half of fighting and killing and pain, she just doesn't start by telling them, and then try and figure out how to protect Dawn.
I am no sure I make sense, I'm struggling to express some thoughts in english. Sorry.