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