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.
Not really. Do you know how many proprietary, company-specific extensions and modules there are of the Linux kernel out there?
Loads of companies choose not to contribute their stuff back upstream. I don't know why the NSA did originally in the case of SELinux, but I would guess it had to do with transparency, national defense and not carrying the burden of a module / fork solo. They were also not the only contributors even early on, according to the Wikipedia page
Also, if I recall correctly, there was no other option for MAC back them (no AppArmor or Tomoyo).