The leader of France's militant CGT union federation, Philippe Martinez, dubbed the decision "scandalous and unjust" and claimed that the "whole union movement is under attack", following the court's decision Tuesday.
"This is a tough message - and not of the right kind - that the government is sending to people fighting to keep their jobs," Martinez told LCI television.
Goodyear itself and the two managers who were held in the factory in Amiens, northern France, for 30 hours decided not to press charges, the CGT leader pointed out.
"Only the public prosecutor decided to call for a prison sentence," he added. "He is the government's spokesperson and the toughness of the sentence can lead us to believe that the request was made at the highest level - at the Elysée [presidential palace] or Matignon [the prime minister's office]."
The incident occurred in January 2014, following six years of increasingly bitter relations between management and workers at the factory, which employed 1,143 people before its closure.
The defendants, who say they will appeal, argued that they acted in a period when emotions were running high and that the two managers were not physically assaulted, although one was said to have felt humiliated when his ear was tweaked.
Economy Minister Emmanuel Macron declined to comment on the sentence - the first jail sentence for trade unionists involved in a dispute in recent years - but the local Socialist Party branch condemned the verdict and Socialist MP Yann Galut declared himself "shocked" and called for the courts to be as tough with wayward bosses.
A number of bossnappings have taken place in France in recent years, leading to calls on the right to tougher legal action, and employees of Air France face legal action over a protest where one of their bosses' shirts was ripped from his back.