What I don't get...

[Ryllharu]

...

It appears that the nuances of that sentence eluded you.

"There is a reason most people drown it in sugar and lighter" ::
Starbucks coffee is burnt, and most people don't like the harsh and bitter taste of burnt coffee.

", and why you often can't find the price of a regular black coffee on their menus." ::
Starbucks expects customers to either buy their specialty mixed coffee drinks, or load their coffee up with sugar, lighteners, and added-cost flavorings, so they often won't even put the price of black coffee on their menu. They found people don't really care for their burnt coffee taken black.

The comma separates the two clauses of the sentence, implying a related, but different subject. This would be the equivalent to a verbal pause. Coffee is the subject of the first clause and Starbucks is the passive subject of the second clause. Had I reworded it to where Starbucks was the subject in both sentences, a semicolon would have been the most appropriate.