Earlier quoted context omitted.
It is a freedom of speech issue, at least from a US perspective. Whether newspapers can be held liable for inadvertently publishing false information was the exact issue at hand in New York Times v. Sullivan, a case which is one of the foundations of modern First Amendment jurisprudence. It held that, in almost all circumstances, they cannot.
I hope the fact that this happened in France doesn't throw a wrench in the reasoning. Inadvertent implies no intention, accidental. Bloomberg did zero due diligence with regard to this information, no attempt to validate it or the source was done before publishing. This resulted in market manipulation and that is punishable by law. Newspapers could always publish anything by just saying "I found it on the internet".…
humanrebar wrote that if it had hypothetically occurred in the US, it would "push against the First Amendment". You quoted that phrase and replied that it "isn't a freedom of speech issue", which I interpreted as disputing the claim that it would hypothetically push against the First Amendment. I replied in that context.
Whether it violates freedom of speech in a more abstract sense is, of course, subjective. I think that requiring the press to do some degree of due diligence is not unreasonable, but I also think there are advantages to the US's more deferential approach.
As for the Press Freedom Index, I'm under no illusion that the US is perfect in that regard.