"I can't find any necessary justification for such a bill to exist"
Because for a smaller company it costs a ton to let someone go if they don't work out for the company.
"Roubaud knows that firsthand. In his 33 years as head of Simonet, he said, he has fired only four people. Twice, the dispute involved years of legal wrangling; in one case the costs matched his company's annual profit."
It helps lessen all these bullshit legal procedings if someone doesnt fit the mold of the company they cant sue if they are let go.
"the contract not only discourages companies from hiring and firing, it also discourages employees from leaving jobs voluntarily - not least because severance payments that increase with seniority make it advantageous to be fired."
A company is more likely going to hire you and let you work there for two years to see if you fit before offering you a full time job, than if they had to decide in 3 months whether you fit the job. At that point in order to fire you they would have to offer you a years pay or so.
This bill allows companies to hire more people "risk free" in order to weed out suck employees and find some good quality people.
So this bill is actually creating more job for younger people. Jobs that companies would not have offered before if they had to make a decsion in 90 days.
It takes some of the risk out of hiring younger people at small companies which could be ruined if someone takes them to court for serverance pay because they were let go.
EVERY country EVEN CANADA has this type of law, some are longer than others.
http://laws.justice.gc.ca/en/L-2/199680.html#rid-199727
^ canadians have 90 days, if a company wants to let you go after 90 days they have to give you a minimum of 2 weeks pay.