You can read my article, or Drew Devaults on why he doesn't trust signal, which get more into this, but the short version is that US security forces don't have time to read the content of everyone's message anyway, they care more about the metadata: message timestamps and social graphs.
Signal stores all that data (via required phone numbers, meaning its linked to your real name and address), and via the US's key disclosure laws, it would be illegal for them to tell you that the US government is hoovering up that data.