10 Nissan (Nasso)

Why is a Sotah rewarded (Bamidbar 5,28+Rashi) just because she wasn't mezaneh?She still did the avaira of yichud which is assur mideoraysa here because she's married! Why isn't she choteh niscar? Also she didn't actually do anything to get rewarded! We can't just reward her for doing an avairah!