Semantics.
"Premium tickets" makes the purchaser feel as though they're paying good money for something and getting something in return. Perhaps it's a great view/ideal location, aisle seat, tough to get ticket, etc. In reality, we know that the word premium means basically nothing except that the show decided to charge more but not everyone is aware of that.
Calling something "top price" just makes it seem expensive/overpriced. Also, based on how the Broadway League calculates grosses, if a show didn't call their highest priced tickets premium, their gross potential would be higher and their gross potential percentage would be lower.
It's all marketing, folks.
Scratch and claw for every day you're worth!
Make them drag you screaming from life, keep dreaming
You'll live forever here on earth.