The law mostly. Otherwise ownership of companies would be mostly pointless, similar to that of gambling (in that your fate would be mostly random, more so then it currently is).
Your statement about the law is for the most part not true. There's a much more in-depth discussion of this at this older HN posting: http://news.ycombinator.com/item?id=3227980
I agree with you that yahoo should have sold to microsoft, by the way. But the legal duty to maximize value bit is just not accurate.
http://en.wikipedia.org/wiki/Board_of_directors#Duties