In some cultures customs become laws,is this logical?

This photo is from Russia Today.I thought it was the Radio Times for a few minutes

Why  or when should a custom become a law? Why would it be necessary? If it were the custom to eat Christian missionaries in some remote African tribe,ought that to become a law thus ensuring their descendants would cry:
But mother I don’t want to eat a Christian missionary,I want some green leaves and a banana!
And as many of the missionaries were celibate the supply would run out hence it follows that these poor folk would have to build ships and sail to Britain to find some more as they were common here once but now only a few are left in the wilds.
Would you agree that customs might  make bad laws and which of your customs would you love or hate to become laws?