How do CS graduate school admissions weigh an unrelated master? I am a Computer Science graduate living at a developing country and I have two options to choose,

Option I:
Pursue CS masters at a reputable, but not leading, national university with a limited alumni, a few of whom in the past has CS Ph.D. admissions from top 40 US graduate schools.

Option II:
A very prestigious scholarship provides me an opportunity to pursue a one year master's in sociology at a well known UK university. Then I can still continue with Option I. This will have a positive impact on my career at work too.

My final goal is getting a CS Ph.D. from a US university, but option II allures me. Will Option II have a negative effect on my future career from the perspective of an CS admission committee member ? Is it viewed as a total waste of time ?
If your final goal is to have a CS Ph.D., getting a masters in sociology first will in no way help you reach that goal.  Doing option II may be viewed as a "waste of time" by CS admissions committees, though it probably won't preclude you from getting into a masters program afterwards.  I'm guessing the net effect will be slightly negative from the point of view of a CS admissions committee.

However, it doesn't mean you shouldn't do option II, especially if you think you'll enjoy it.  You might even find your career plans changing after getting a masters in sociology.