It appears to me paros are specialized shrimp-killers. I have the same experience you described: not a single young shrimp can make it to adulthood, although there are a lot of adult shrimp carrying eggs. The way they move and approach shrimps looks dedicated for hunting young shrimps.
Indeed, from an adult shrimp a paro could harvest a lot of eggs, so it would earn more food than when he would eat the hole shrimp.