Suzaku is a viewed as a traitor because he sides with the occupying forces of his country, and that is generally viewed as traitorous, despite whatever noble intentions he has.
Lelouch isn't a traitor, since despite being a prince, he was shipped off and exiled to Japan and then later Britannia just decided to attack it, not caring about Lelouch's welfare or the possibility that he would be killed by the Japanese. He doesn't really owe Britannia any allegiance at all.
I don't like Suzaku. To use /v/'s term for it, he is a moralfag, and I hate characters like that. People with an overwelming sense or morality and justice get on my nerves. Suzaku being a moralfag and apparently profiting greatly from his blind loyalty and honor code infuriates me all the more.
Though I'm starting to see a change. He is now in a position to enjoy all that the Britannian Empire has to offer. He is close to a penultimate position of power, and I think this will change his thinking. If he ever just drops the act and says "You know what? I just want more power, fuck changing the system, now I want to own it," then I will love him and hail him as a wonderful character.
Until then, he is on notice.