Whys is Snape allowed to continue spying?

[Chris]

No: HPFGUIDX 131473

   Perhaps this has been discussed, I'm not sure. During my re-reads a 
question has popped into my mind. Why is Snape allowed to continue to 
get close to Death Eaters like Malfoy? Voldemort obviously knows that 
Snape has betrayed him and joined the Order. Luicius Malfoy more than 
likely knows that Snape is a traitor by now? How can he continue in 
his function as spy when everyone knows he has joined DD. Or am I 
missing something?

Chris
labmystc