The Terms of Service (http://stackexchange.com/legal) currently prevents posting answers only containing a hint: Section four states that:

Under no circumstances will Subscriber use the Network or the Service to [...] (d) knowingly post any false, inaccurate or incomplete material

A hint is technically incomplete material, so a lot of users (including myself) are in violation with the Terms of Service. I think that this should be fixed in the ToS.