These two events are not really equivalent at all. Intentions are key. I'm no apologist for US imperialism/exceptionalism but the US airstrike was not intended to kill civilians and they have taken responsibility. Assad's sarin attack on Khan Sheikhoun, on the other hand, manifestly was intended to kill civilians and the regime is desperately throwing up chaff and misinformation to evade accountability. Moreover, the bright line between the use of conventional weapons and the prohibition on the use of chemical weapons is not a contrived formality rendered meaningless because they both end up creating dead people. Turning a blind eye to the use of chemical weapons against international prohibitions is a very slippery slope that could lead to the normalization and proliferation of the strategic use of these weapons, which — unlike the use of high explosives — presents unique threats to civilization on a mass scale.