The reason some people put an olive in or on a tamale is because of a Christian Belief.
Christian believers believe that when you put an olive on or in a tamale that the olive in a tamale is considered a metaphor or symbol for the Holy Virgin.
The olive in a tamale also represents the Christ child waiting to be born as he is every year at Christmas.
So when you see people putting an olive in a tamale it represents Christ and a Christian Belief.
Not everyone puts olives in tamale but some people do.
I put olives in tamales but not for Christian Beliefs.