The NSA, the original primary developer of SELinux, released the first version to the open source development community under the GNU GPL on December 22, 2000.[6] The software was merged into the mainline Linux kernel 2.6.0-test3, released on 8 August 2003. Other significant contributors include Red Hat, Network Associates, Secure Computing Corporation, Tresys Technology, and Trusted Computer Solutions.
I mean, leaving aside their surveillance tasks, it’s still their job to ensure national security. It’s in their best interest to keep at least themselves and their nation safe, and considering how prevalent Linux is on servers, they likely saw a net benefit this way. They even open sourced their reverse engineering toolkit Ghidra in a similar vein
Ghidra was about hiring and cost savings. Its easier to hire when people already know your tools. Also people are more willing to use your tools rather than expensive ones if they can still use them when they leave (go into contracting). Also interoperability with contractors may improve.
And we’re all the better for it! Needs polish and development of course, but it’s a decent alternative already