Maybe a hot take here but if you're going to engage in a war, whether directly or by supporting an ally with money and supplies, you don't half-ass it. You don't give your ally just enough bullets and fuel to get into the thick of it but leave them hanging when they need to keep going. Whether or not you support the US aiding Ukraine, you have to understand that once that support is given the strategically correct thing to do is to see it through. From the position that we are already engaged in supporting Ukraine, the continuation of that support with the goal of winning is itself justification enough to match the ante in response to your opponent raising it.
A number of factors would make that different. For example if we reached a point where our support started to become detrimental to our readiness to defend ourselves (which, despite arguments from the far right to the contrary, we are not remotely close to doing). Or if Ukraine showed a reapted track record of attacking civilians with our munitions. Or if the war was a losing or lost prospect or this was an escalation on Ukraine's side. But none of those things are the case. Ukraine has not gone out of their way to attack civilians and has in fact fought essentially exclusively a defensive war, they are doing quite well at it and still control their own fates, and Russia escalated to cluster munitions first. This is only a response in kind. With all those factors taken into account, the decision to provide these munitions is justified simply by the fact that they make Ukraine's odds of winning, and winning sooner, better. If Ukraine starts bombing civilians with them then we can discuss whether or not it was the right thing to do. But their track record so far suggests they have no intention of flipping this to an offensive war. Whatever Russian sites they attack on Russian soil can be assumed to be military targets that pose a direct threat to Ukraine and nothing more until proven otherwise.