Because Linus signed off on it. To get that complex change into the kernel, the NSA had to convince Linus that it's a good idea, which can be a next-to-impossible task. I trust him to review the code more than I trust myself.

SElinux is just policy based MAC you can clearly see what the source is doing. Not everything the NSA does is totally evil, sometimes they actually do what they are supposed to do, which is secure US infrastructure and that includes open source projects. SEAndroid is also a good project, and the NSA is just like any other security specialist who likes to work with open software and rejects blackboxes. At least your intel agency gives you something in return, mine spies on us and gives us nothing.

They wouldn't need to purposely sabotage the linux kernel anyways it's like 15+ million lines of code they can easily review and find bugs as a homework assignment in first year internal classes on offensive exploitation.

While this is possible, I believe it'd be extremely unlikely.

Let's say they indeed decided to include something in the code submitted. It's an open source project, and it is a just matter of when someone will catch that. Once someone catch that, it gives them nothing but material evidence that they are up for something there. (and for that matter, it's not that every Linux users will have SELinux enabled to begin with.)