Actually this goes back to the early days of Hershey's. As part of their drive to produce affordable chocolate, their main formulation was developed around slightly spoiled milk, which has an acidic taste. That's why all Hershey's chocolate since the beginning of time has had that 'vomit flavor'. Obviously they're not allowed to use spoiled milk anymore, but they wanted to taste the same as it always has. So they add the acid as an ingredient to replicate the same flavor.

Stomach acid is hydrochloric acid.