The Syrian scenario wasn't the greatest idea either but it's also not the same scenario. Russia physically holds various parts of Ukraine, so it would be a way different result/embarrassment to have U.S. fighter jets or ground defenses shooting Russia planes out of Ukrainian air space which they basically consider their own. It would certainly be an escalation where nobody could predict the next actions.
This
Neither nation was at war with Syria, they were operating as ‘guests’ of different regional forces. Any incident could be chalked up to mutual confusion and brushed over with diplomatic kabuki. Doing it Kyiv during a Russian invasion is undeniably an act of war.
