Well the initial cause of the war, or rather justification for it, was to defend the Gauls against outside invasion and Caeser was given the title "Protector of the Gauls" in that endevour - so he isn't trying to smear the Gauls, he is providing justification for protecting them.

To be fair Rome was concerned that the actions of the Helvetii were not just a threat to the Gauls but to Roman allies and the Roman province of Transalpine Gaul (and possibly Rome thereafter) - but as with many military actions once the war has begun it takes on a logic of its own and so Caeser was soon in conflict with those he had formally been sent to protect - although again the reason Rome appointed a "protector of the Gauls" (and some Gauls were already under Rome remember, Transalpine Gaul stretching into what is now modern day Italy) was because those Gauls not under direct control of Rome were effectively a buffer region between Rome and more hostile forces who, if they invaded, would both directly threaten Roman possessions and could cause a mass refugee event of Gauls into Roman territory.

In that context "freedom loving" (although I confess I don't remember that phrase being used) means not wanting to be invaded by the Helvetii and either stay independent or a Roman province or ally, rather than referring to individual Gauls and their attitude towards the institution of slavery - and while it is the "good reason" for the legions' presence, the "real reason" was to defend Rome, Roman allies, and Roman interests which then turned into punitive expeditions that expanded Rome's domain.

Of course some Romans, like Cicero, were critical of the war as it dragged on partly on exactly this basis that Gauls were seen as a civilised peoples and undeserving of being subject to ruthless Roman expansion.