[1] https://en.wikipedia.org/wiki/Host_Based_Security_System
The 3-letter US agencies do not have direct access to Telegram's servers, they do have when it comes to Whatsapp (or to any other US-based company). The same goes if you're a Russian doing anti-government stuff in, well, Russia, it's better to put your fate in Whatsapp's (and Meta's) hands, presumably the US agencies won't come after you for being against the Russian government. It's a classical game of arbitration (for lack of a better word).
What makes you think so?
(This is actually independent of you or Signal, I just find it amusing that people throw their security brand™ behind some app and it's just them picking the one they like most of the time.)
Tribalism is a helluva drug.
Most likely implementing FSB backdoors.
And even if he didn't, if you run the most popular messenger app in Russia you can't simply hide behind "citizen of some other country". KGB is known for poisoning/shooting people abroad for much less, and especially if you have family or close relatives back in your country of birth you really have little leverage to speak of.
There's nothing for governments to allow people to communicate freely. Telegram is one of the few platforms that allows that... and this is a good opportunity for the US government to do that.