1.) The wiki says that
On 12 September, the Anglo French Supreme War Council gathered for the first time at Abbeville in France. It was decided that all offensive actions were to be halted immediately.
The article notes that the French offensive encountered only light resistance, so even though they had difficulties with mobilization it seems like they'd want to push as far as possible while the bulk of the German army was in Poland. But it looks like that the War Council halted all offensive operations extremely early - why was that?
2.) The wiki also notes that the French were lying to the Poles about their future offensive plans
General Louis Faury—informed the Polish chief of staff—General Wacław Stachiewicz—that the planned major offensive on the western front had to be postponed from 17 to 20 September.
Why were the French continuing to deceive the Poles?
The French government viewed it as mostly an exploratory operation to probe German defenses. Despite the fact that they could have easily broken through the Siegfried Line, they remained cautious and ultimately withdrew because they preferred to force the Germans into the offensive role.
Source: http://fileserver.net-texts.com/asset.aspx?dl=no&id=26455 (bottom of page 4)