In a series of furious interviews, most notably on talkSPORT, BreatheSport and ESPNcricinfo, Pietersen also labelled former England coach Andy Flower "a clown" for dropping Nick Compton ahead of the 2013 Ashes and suggested that the decision to disregard Bell's experience for a tour of South Africa was "a disgrace."
"England are playing the best team in the world with one of the best fast bowling attacks the game has seen for years," Pietersen told ESPNcricinfo. "So not to take one of your most experienced players with a great record against that team is absolutely ridiculous. You have to take him for his experience.
"He's played 118 Tests and you need that experience on tour. He's probably got another five, six, seven or eight Test hundreds in him. It is an insane decision to leave him out.
"Yes, Belly hasn't scored the runs he would have liked of late. But that happens to all players and it's at those times that the management need to stick by him. It's a disgrace they way they've dropped him now."
Pietersen also questioned what he saw as the inequitable treatment of Bell compared to the patience shown to the team's captain, Alastair Cook, and questioned what he sees as an ageist attitude prevalent within English cricket.
"Captain Cook himself can go two years without a Test hundred but he can play every single game," Pietersen said on BreatheSport. "As for Belly, he struggles a bit - he got a hundred, what nine or 10 Test matches ago? - but he's not good enough to get to the South Africa tour? I think it's pathetic and I think he's been hard done by here. People need to start having a hard look at themselves.
"He's 33, not 53," he told ESPNcricinfo. "Why is that, in England, that 33 is considered too old? In other countries we have seen the likes of Jacques Kallis and Ricky Ponting play on far longer than that. Belly isn't over the hill. Of course he can fight his way back from this. I believe he will. He is a class player. You don't score 22 Test centuries without being a top class player. I'm very angry about this."
While Pietersen might be expected to be outspoken on this issue - as has been well documented, his relationship with the England management is not cosy and he is a director of the management company that looks after Bell's interests - he has not been alone in his criticism of the squad.
Mark Boucher, the former South Africa wicketkeeper, tweeted that South Africa would experience "a boost in confidence" by seeing a squad without either Pietersen or Bell, while Graeme Smith, the former South Africa captain, tweeted that he was "very surprised" Gary Ballance had been recalled and that bowlers would be "licking their lips" in anticipation of bowling to him.
"I know the South Africa players, and they will be licking their lips bowling to a bloke like Gary Ballance," Pietersen told talkSPORT. "You just have to see what the likes of Mark Boucher and Graeme Smith have said on social media today, England are just being laughed at. It's horrendous. Goodness, my mind just boggles when I see what happens there. It's frustrating."
He did, though, see merit in one of the decisions. "It's good to see Compton back. He was dealt an horrendous blow by that clown who coached England a couple of years ago."