What "populace" are you talking about? The Gauls? The Gauls didn't have much of a choice, they were a subject people one way or another. The Romans? A promagisterial governor had the right to do just about whatever he wanted during his five years (in Caesar's case ten years, thanks to the renewal of his proconsular imperium at Luca in 56) in his province. Egregious breaches in the law, like plundering allied cities (which is one of the things Cicero attempted to prosecute Piso for) might result in a recall to the city, but even then the result was not certain, and a promagistrate had every right to ignore the senate, which held no actual power to stop him--Gabinius ignored his recall until he had finished with his affairs in Egypt. Caesar's wars were hardly unique--only about a decade earlier, Pompey had used his extraordinary magistracy, giving him imperium to put down the pirates, to march all throughout Asia and Syria stomping anything in his way, for no particularly good reason--the entirety of the eastern provinces that would be confirmed when the Eastern Settlement was passed in 59 were captured by Pompey in those few short years, often without much justification whatsoever

Caesar nevertheless justified his wars, stressing that he was defending the province or assisting allies. The initial incursion into enemy territory was in defense of the migration of the Helvetii, who had abandoned their homeland and were threatening the Gallic tribes just beyond the Roman frontier, as well as bringing the possibility of incursion into the province itself. How real a threat this was is debatable at best, and certainly it might have been handled more peacefully (or at least attempted), but it gave Caesar an initial excuse to march to Geneva, to defend the Rhone valley, which was a part of the province that the Helvetii would have to cross through. Gaul was not a politically-united state, and the Gallic War is really a series of wars between different political entities within the territory--each of those wars received a different justification. The war against the Germans, for example, was begun in defense of the Aedui, a tribe of Roman allies whose enemies the Averni and Sequani had summoned Ariovistus with his Germans to help in their war against the Aedui--Caesar's march north was therefore in defense of allies. The war against the Nervii began because the Belgae assembled an army, supposedly with the intention of attacking the Roman army assisting the Aedui. The war with the Suebi began because the Suebi attacked and massacred a body of Romans while Caesar was busy settling some Germanic tribes that the Suebi had forced across the Rhine--this was under truce. During this war Caesar was accused by Cato of violating his imperium as a promagistrate, since by crossing the Rhine, Cato argued, Caesar violated the boundaries of his province (the same offense for which Gabinius was tried and acquitted after marching his army into Egypt from Syria, his province). The British expeditions were pretty poorly justified--ostensibly Caesar wanted to stop the Britons, whom he claimed (probably rightly) to be harboring fugitives from the Belgae, from assisting the Gauls. The war against the Veneti was fought when the Veneti entered into rebellion and imprisoned Roman envoys. The revolts of the Nervii and other Belgic tribes that Quintus Cicero handled so stalwartly were, well, revolts. And of course Vercingetorix's revolt was...a revolt.