It is also why the US forces had large US flags painted on the sides of their vehicles, the theory being that if the defenders knew that they were facing Yanks and not Brits, they'd be less likely to fight them. Didn't quite work out that way, but it was worth a try.

The big reason for it was that the UK was not particularly well regarded by France in 1942. First, the British abandoned France in its struggle for survival and returned to the UK to take care of itself*, resulting in French capitulation to the Germans, then they went and sank the French fleet which was in harbour in Mers el Kabir in July 1940. That was part of Operation 'Catapult', they neutralisation of any French ships the British felt they could reach. Since the French were felt to be at risk of having their ships seized by the Germans at any time, the British position was better to have them at the bottom of the sea than in German hands.

The French position was a bit different. They had an armistice agreement, it did not include handing over their ships to the Germans, and they, being men of honour, (unlike those perfidious Brits), would ensure that their ships would not be taken. When asked to surrender, the Mers el Kabir fleet declined and got sunk for their troubles, the French lost three battleships (one permanently), four destroyers damaged, and over a thousand dead.

Slightly more successful negotiations in Alexandria saw the French at least self-disable their ships.

Then, to add insult to injury, the British attempted to land forces at Dakar in Sept 1940. A spirited engagement, which saw both the British and French battleships present (Richelieu, in harbor, and Barham, at sea) take heavy damage, saw the British and Free French forces repulsed. Again, the French had an agreement, they had their sovereignty, and they would defend it against everyone.

At this point, the way the British were acting, with friends like that, who needed enemies?

So seeing British forces, yet again, show up at French African territories was not going to be well recieved. The Vichy French defended their territory, perhaps unwillingly, but out of duty, until finally the local French commander, Darlan, could be found, negotiated with, and agreed to switch sides.

The irony of it all is that when the Germans did, finally, decide to try to capture the French fleet at Toulon, the fleet scuttled, as French honour demanded.

*Of course, that wasn't what happened, but to this date that is a common enough perception in France. In 1942, one can imagine that it was a very strong perception.