The England and Wales Cricket Board (ECB) management met with Pietersen earlier this week informing him that he was not in their future plans.
Swann was on tour with Pietersen during the Ashes, although he retired from all cricket following the Perth Test.
Writing in a column for The Sun newspaper, Swann revealed said that he was very surprised at the decision as he had seen nothing to indicate that there were problems between Pietersen and other members of the touring party.
"Myself and other England players were not exactly members of his fan club during the Textgate scandal of 2012," he wrote.
"But I changed my opinion of him once he was reintegrated back into the England team towards the end of that year.
"He made a huge effort to improve his attitude around the dressing room. I saw or heard no issues with him in Australia this winter, his approach was exceptional.
"That's why I was baffled on Tuesday when he was effectively sacked as an England player."
Swann played 60 Tests for England, taking 255 wickets before deciding to call time on his career after the Perth Test.
England went on to lose the final two matches of the series in Melbourne and Sydney, where it is rumoured that Pietersen and captain Alastair Cook had a disagreement.
"Clearly, Kevin must have upset people enough for the England hierarchy to decide he is no longer wanted," Swann added.
"Don't forget we're not talking about a middling player here - we're talking about a batsman of sublime talent.
"As I say, I saw nothing whle I was on the Ashes tour. His approach was spot on.
"Until the powers that be shed some light on the situation we are left only with conjecture and rumour.
"I'm sure every cricket fan would like to know more detail because Kevin is the player more than any other who puts bums on seats.
"Let's hear the truth so all the speculation disappears."
England's loss could well turn out to be the gain of Twenty20 leagues around the world, to which Pietersen is now available.
Speaking to the BBC Radio Five Live last night, former IPL Commissioner Lalit Modi said he believed Pietersen could earn as much as £2.5 or £3 million by being available for a full IPL season.
Pietersen has previously played for the Royal Challengers Bangalore and the Delhi Daredevils, who would be able to match any bid made on him and re-sign him under the new rights to match (RTM) clause brought on this year - provided they have not already used their three RTM options and they have budget available.
The auction is on 12th and 13th February in Bangalore.
© Cricket World 2014