Why do crows sit on treetops even when it is cold?
Birds (crows inclusive) have better heat conservation adaptations as compared to humans. Some of these are (but not limited to):

-Hollow tubes in feathers which provide insulation reducing heat loss. 

-Reduction of appendage length especially in species that dwell in cold regions.

So conditions we perceive as cold are relatively warm for birds.    


  My first guess was that it isn't cold enough.


From this question on biology.se I can say it could be warm for them as it addresses birds surviving at -40C. Plus some of the answers and comments on the same question cover a couple of adaptations that birds exhibit to strive in such conditions.

edit: links that provide additional information on the same

-http://toughlittlebirds.com/2012/12/26/how-do-birds-keep-warm/
-https://beyondpenguins.ehe.osu.edu/issue/arctic-and-anarctic-birds/how-do-birds-stay-warm
-http://birding.about.com/od/birdingbasics/a/howbirdskeepwarm.htm
-http://gardenwalkgardentalk.com/2013/02/19/how-do-birds-keep-warm-in-winter/