Alright, well in the interest of a conclusion, I'll say that you made really good points, I've changed my opinion on some but not all of the topics.
> "they want to know who's downloading tails, who's using signal, who the source of some journalist is"
They don't care about random people doing those things, but if someone with a known terrorist cell association is in the US talking over signal. Or if someone is visiting extremist sites using tails, they'd want to know (and they can using metadata available today). They're not interested in home-grown terrorism or law enforcement, but all other matters of national security don't neatly fall into "foreign vs domestic" buckets. Even if it is all happening outside of the US, the servers Signal uses might be in the US for example.