The footage also shows that Spaniard Alonso, who was similarly reprimanded by the stewards, stopped on the racing line, causing two cars including Lewis Hamilton to take evasive action.
As it was Webber's third official reprimand this season, the penalty carries a ten-place grid demotion for the next race.
The 38-year-old said the penalty is "tough to take".
"Just spoke with Mark Webber," Sky commentator Martin Brundle wrote on Twitter. "You have no idea how p***** off he is".
Red Bull boss Christian Horner also disagrees with the penalty, insisting the sight of a stricken hero getting a lift back to the pits is "good TV".
Hamilton also thinks it was a good spectacle, but the Briton admitted he was "shocked" when he turned a corner on the slowing down lap and found Alonso stationary on the racing line.
"If Mark had been walking across where I went then I would have run him over," he said.
"It's good for the fans to see and, as long as it's done in a safe manner - you don't stop on the racing line - then maybe it should be allowed for the future."
But veteran F1 correspondent Roger Benoit, writing for Switzerland's Blick newspaper, said that after reviewing the video footage, Alonso and Webber had been clearly "reckless", behaving in a "potentially life-threatening" manner.
Defending the penalty, Swiss steward Paul Gutjahr insisted: "We could not do otherwise."