I think it's more likely that the rebels used them on their own people so they get the US to involve and support their cause.
It's a possibility.
Another possibility would be that the regime actually thinks it can get away with it since Russia and China are blocking the UN mandate, which prevents the US from an intervention.
If the US goes to war regardless of the UN mandate it means they are breaking international law, and if even the US ignores these laws then what stops other countries from doing the same thing? It would have a pretty bad impact on world politics in general. It's a very slippery slope.