We didn't tell them, they penetrated our intel and command services. The KGB didn't go anywhere...it just changed it's name. Every nation has it's own agenda, I for one will never trust the Russians.
In addition...when it comes to national security I only trust the USA. Even our closest friend the UK collects on us. It's how the game is played, trust no one but your own, your objectives are the only ones that matter, if they coincide with someone elses, well than thats just peachy. If not, oh well.