They can't do that.
It can only compel the release of collected metadata to agencies.
It's great that you can make claims like this based on zero evidence or even allegations, but there is no basis in fact for it.
What's more the number of 3rd party people that have to be involved in something like this make it virtually impossible that the national security letter structure could keep them all silent, especially since there are numerous foreign nationals in the supply chains.
Go read the huge amount of ACLU coverage of these, or the many articles linked, or the congressional testimony. There are no allegations that this mechanism is being used to do what you say.
That doesn’t prove they have done it. But arguing they can’t is crazy.
Care to state your belief about the relations between the TLAs and the larger networking and communications firms in the US?
Not the OP, but I think that addressing specific parts of the claim is extremely important.
We've seen this around the PRISM program, where the allegations of support by tech companies were confused by their support for lawful law enforcment warrants (as opposed to the NSA's dragnet surveillance via PRISM). This confusion has reached the point where I saw a lecturer claiming Google helped the NSA collect data as part of PRISM, where the NSA's own slides[1] show the opposite[2].
[1] The famous "smiley slide" https://arstechnica.com/tech-policy/2013/10/new-docs-show-ns... or https://slate.com/technology/2013/10/nsa-muscular-program-sp...
[2] https://arstechnica.com/information-technology/2013/11/googl...
Even in China, they might not need to rely on explicit state security authority, just tie it up with state sponsored funding or other softer measures.