US supreme court
"Supreme Court", Guardian. It's a proper noun. We don't call your stuff the "UK house of commons", and that isn't a difference between British English and American English.
And it's not a typo, because you consistently did it through the whole article.
justice department
"Justice Department". Well, really, "Department of Justice", but whatever, people use that informally.
sixth US circuit court of appeals
"Sixth Circuit Court of Appeals".
How is it that you stripped caps off of every proper noun referring to a government institution in the entire article, but still managed to capitalize "Starbucks" and "Workers United"?
EDIT: And they do it in another article too:
https://www.theguardian.com/law/2024/apr/24/us-supreme-court-abortion-rights-emtala
For the second time in a month, the US supreme court on Wednesday heard arguments in a major abortion-rights case, the second to reach the justices since they overturned Roe v Wade two years ago.
But there they do capitalize the FDA in that article:
Food and Drug Administration
EDIT2: And they did capitalize the NLRB in the first article, so not every government institution:
the US National Labor Relations Board (NLRB)
EDIT3: And their counterparts at The Telegraph and the BBC are capitalizing the proper nouns for the same institutions:
https://www.bbc.com/news/magazine-33103973
Who are the justices on the US Supreme Court?
The US Supreme Court will rule on Monday on whether cities can fine people who set up homeless camps in public spaces.