This is on their site:
Again, security issues have been reported, but nothing malicious like a backdoor -- we would argue that it's only uninformed conspiracy theorists that speculate about deliberate backdoors in Tor these days.
There have been many NSA revelations the past few years. Just recently, it's been confirmed that the NSA even has spies that have infiltrated in tech companies. We know that they have actively been making use of undisclosed security holes in software and that they actively try to put them there.
It seems to me that it's very naive to assume that intelligence agencies don't have agents operating as contributors to the Tails project. Even if they aren't interested in drug busts or cp busts, it's very clear that they have an incentive to infiltrate the project. If a drug dealer can hide his identity with Tails, so can a terrorist.
Are they trying to or have they already done so? Only they know.
I don't know how one would weed out these infiltrators or their contributions. What I do know, is that taking a naive stance towards this issue is NOT a solution. They're making some big assumptions, and we all know that assumption is the mother of all fuckups. The audit history (https://tails.boum.org/security/audits/) confirms this naivity.
Tails is open source - kinda hard to inject backdoors into it.
Even more - it's based on debian live-cd system, the whole codebase is pretty small (I looked through it) and very intuitive lowering the risk of backdoors lurking everywhere.
There are a couple of tails-flavoured packages like the TorBrowser which includes their own patches but everything is still visible.