In reply to drunken monkey:
According to people who seem to know a little about it the problem is that when a person is trained there is no way of knowing what they really are nor who they may decide to fight for at a later date... seems a fairly reasonable supposition to me.
Similarly arms that may be supplied, in Turkey or Jordan for example, may be intended for "nice chaps" but what guarantee is there that they will end up with them? Or more simply as a lot of the fighting now is between different islamist groups or between islamist and the ever diminishing secular anti-Assad people (if they ever existed really) what happens to the arms of the losing groups when they are defeated by stronger people? As at present it seems to be ISIS and similar who are coming out on top then it seems reasonable to suppose that the weapons will end up in their hands too... as you say yourself.
IS seems to be on a roll at present and grabbing everything, from weapons to banks and even oil fields, using the weapons from Syria in Iraq, and maybe visa versa according to the way the tide changes.
One "easy" step might be to put pressure on Turkey and Jordan to shut their frontiers and stop helping the rebels in Syria, but doing this would help Assad and the Western powers (power might be more accurate) don't want to do this, they don't want to admit they are wrong in Syria any more than they clearly were wrong in Libya.