My understanding is that up until the Arab Spring kicked off the civil war in Syria, it was reasonably stable and prosperous. Even if the guy in charge is a tyrant and a dictator, people usually don't pick up and leave when they aren't likely to starve or get shot. Now Syrians are very likely to starve or get shot, so they leave. Not to mention a rather high chance of ethnic cleansing if the wrong groups end up on top.
Blaming Assad is way to demonstrate they aren't supporters of his regime without claiming affiliation with or opposition to any of the rebel groups.
The US shooting down Russian planes might not lead to WW3 but it'd be a very unstable circumstance. Presumably both sides are smart enough not to escalate, but it's not smart to rely on that presumption.