His Legend could be awkward for the government or ONI if he was to take a stand against them. He doesn't need to betray them, he just needs to voice a different opinion. His legend gives his opinions weight. So his legend has to be undermined.

Locke would have been lied to but he cares about the truth. If he had reservations about John's "treachery" then he would hunt for the truth. He would hunt for the real John.

I wouldn't want to spend 3/4 of the game trying to take down the Chief only to work out that the Chief wasn't a traitor until near the end of the game. We know the Chief isn't a traitor so spending too much time as another character who hasn't worked that one out would wear thin pretty quick. Just like it was annoying to play as the Arbiter and to take forever to work out that he was being played.

I would prefer if Locke had some inkling or reservation early on and was seen to be open to the truth and interested in finding out for himself. That way I wouldn't feel that I was just treading water, shooting things, until the big reveal finally happens.