It was revealed on Tuesday that Pietersen's international career had come to an end after he was dropped for the tour of the West Indies and the Twenty20 World Cup in Bangladesh.
Stewart, who regularly works with Pietersen in his role as Surrey's director of cricket, believes that more information about the decision should be released.
"At times you could call him a maverick cricketer, but he's a match-winner, a natural born winner and a game changer," Stewart told Sky Sports News. "All these types of things create a tremendous cricketer.
"I do not know the full facts but as an England fan I'd want my best players playing for England, to give us the best chance of winning.
"I'd be asking who has made these decisions and why have they come to those decisions. All Kevin Pietersen has ever had at Surrey is a very positive influence."
Pietersen represented England in 104 Test matches, 136 one-day internationals and 37 Twenty20 internationals