Tories' bill allows them to unilaterally change the NI protocol to reduce the number of customs checks. Some goods will just be waved through between NI and GB. In a nutshell, this means the UK will not live up to the Brexit withdrawal agreement. EU says they will retaliate with sanctions. US also officially discouraging the UK from breaking intn'l law.
This last bit, "breaking international law", this seems tenuous. I don't anticipate there being any specific international law that refers to the relationship between the UK and the EU. Breaking agreements between the UK and the EU isn't necessarily breaking international law. But I'm no expert on international law.

With that said, I fail to see what this has to do with USA, other than an economic interest.

The NI situation has always been difficult. I'm of the opinion that it would be better for all parties if NI was part of the Republic of Ireland, not part of the UK. It's a colony, and not a colony like the Falklands where people settled on uninhabited islands. It's a colony where we took industrially productive land off another country. NI unifying with RoI would solve the EU problem. It would, of course, create problems with unionists who don't want to be part of the RoI, but so long as they have the choice to come to the mainland UK, then I'm ok with it.