First of all, the US is not supporting ISIS (or ISIL or IS, as it's also called) in Syria. The presence of extremist elements among militant groups in Syria is one of the major reasons why the Obama administration has been so cautious about giving weapons to Syria rebels. This concern seems to be widespread among the US political leadership (for example, it was one issue were Barack Obama and Mitt Romney mostly agreed).
It could be asked if, by supporting other rebels, the US is indirectly supporting ISIS, since the rebel groups all oppose Assad. Considering the violence that has occurred between the various rebel factions, it would be difficult to argue that (for example) supporting the Free Syrian Army against ISIS is tantamount to supporting ISIS.
It should also be pointed out that just as there are numerous rebel factions in Syria, so too are there numerous anti-government factions in the areas of Iraq where ISIS is active. It's often unclear how much strength ISIS actually has and how much of the rebellion is controlled by these other militant group.