Why do chickens continuously lay unfertilized eggs?
Chickens are domesticated Red Jungle Fowl. In the wild, Red Jungle Fowl live in flocks with one rooster for every few hens (where "few" is probably less than five in the wild, though in zoos flocks may be larger and have a higher rooster:hen ratio).  Since one rooster can fertilize many hens, in the wild virtually all eggs will be fertilized.  Since jungle fowl, of course, lay far fewer eggs than domestic chickens, the cost of the extremely rare unfertilized egg will be low. 

In principle, there could be a molecular mechanism that checked for evidence of fertilization before allowing an egg to be laid, but of course such a mechanism would have its own costs, and would almost never actually be useful.