How does Snape get away with it?

[augustinapeach]

No: HPFGUIDX 83216

Ever since finding out that Snape is spying on Voldemort for 
Dumbledore, I've wondered how he can get away with it.  In PS/SS, 
Voldemort was on the back of Quirrell's head while Snape was warning 
Q to figure out "where his loyalties lie" (quote may be wrong, don't 
have books with me here).  Even through that purple turban, he was 
bound to have heard the conversation. Then in GoF, it's likely 
Crouch Jr., posing as Moody, overheard sensitive information that he 
passed on to Voldemort via the eagle owl.  So it seems to me that 
Voldemort would know where *Snape's* loyalties lie.  All that makes 
me wonder -- what information is Snape giving to Voldemort et al to 
convince them he is still a loyal DE and that makes him useful 
enough to keep alive?  Is he being set up for a scene in which V 
reveals that he knows Snape is a double agent before killing Snape?

Any thoughts?  (Sorry if this has come up before)

AP