Yeah, but it has to happen. There's supposed to be rivalries and confidentiality among guilds. If the Imperial Legion knew exactly what the Thieves Guild was up to, well, the Grey Fox would have made a very nice scarf for Heironymous Lex.

Well I am sure there is a Stormcloak member in the thieves and mages guild
Probably some of the mages support our ideals. Who know until we play the game.

Perhaps the leaders of Guilds can limit member access. So if I was to join the Thieves Guild to boost the economy and what not, my access could be limited so I could only see so much of their information. I could be cut off from member names and numbers etc...This way it stays realistic. It would actually be more realistic this way.

Or, you could just be a guard being bribed by the thieves. Didn't you join the legion first anyways?

I suppose it doesnt really matter. And yes I joined the Legion first