Dude, we arm them everyday. Sometimes on purpose, sometimes on "accident."
Only "moderates" got armed, but I'd like to know how they tested who was and wasn't moderate.
We actually don't arm Daesh on purpose, but if you can find a valid (valid) source that proves otherwise, I'm all ears. A current, valid source. And no, Turkey's involvement does not count (because I already mentioned that).
Moderate rebels were tested for a number of things. First, they had to prove that their group was nominally secular and democratic. They also had to sign a waiver agreeing not to attack the Assad government, only to attack Daesh. They also have to be part of certain MOCs, usually run by people outside Syria. The Southern Front (largely secular) is supported mostly by Jordan and a few others, whereas the more Islamist factions near Aleppo are run by Turkey (Idlib governorate is traditionally way more conservative than the rest of Syria).