This is where the 'many eyes' things comes into play; if the whole distro is OSS, then you can be pretty sure that it's good.
Debian, which is much better known and in much wider circulation than Tails generated weak SSH keys for two years. Yes, it was indeed very big news. When it was found. After two years.
Oh, and tin-foil-hat on: Do we know (actually know-know, not just assume, think, trust) that the weakness wasn't planted there?
Also, how do you know that Tor and Tails aren't infiltrated by the enemy (for any value of "enemy")?