Perhaps they were just 'enchanted' by Sauron. I mean, the One Ring had a will, so to speak, in that it did the work of its master, even when not on his finger. Likewise, Sauron could just have 'put a little of himself' in the Watchers and left that to operate independantly.
Of course, the question remains as to whether they were placed by him, or set as watchers by the men of Gondor. They might not be inherantly evil, but serving whoever owned the tower.
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