The US entry into World War I was justified. The Germans resumed unrestricted submarine warfare and sank 7 US merchant ships. The US can't just sit there and be attacked. That's why the previously dovish President Wilson and and overwhelming majority of Congress went for it.
but why were we only shipping things to one side of the fight? If we were selling things to both sides, the British would have been sinking our ships too, should we have gotten in a fight with them?
We did trade things with both sides. When the Germans sent some "merchant submarines" past the British blockade, we traded with them.
But in any event, the British boarded or turned away US ships that tried to trade with Germany. The Germans sank ships that to reach the UK, resulting in loss of life. The US obviously didn't like that. This was not because the Germans were morally worse than the British, it's because they had the smaller Navy and did what they had to do. But the US certainly didn't like having its citizens die.
When Germany resumed unrestricted submarine warfare in 1917, they knew it would lead to certain war with the US, but gambled that they could win before US might was brought to bear.