You said it is no accident a lot of weapons of ISIS have 'made in america' engraved on them, but it seems they didn't got them intential. They did got them by accident.
ISIS seems to obtain most of its weapons from capturing US-allied Iraq military and from the Syrian rebels. It is a complicated issue though, as even though America and other western countries may not directly be supplying ISIS, they are aware that a large number of the weapons they export to these countries ends up in their hands. This was an interesting read on the subject: http://edition.cnn.com/2015/12/08/politics/amnesty-international-isis-weapons-u-s-/