This [0] may provide a hint. Heimdal was developed outside of the US and not subject to export restrictions, unlike MIT. So perhaps in the beginning it wasn’t the package of choice to begin with.
And this [1] says for interoperability reasons.
[0] https://docs-archive.freebsd.org/doc/11.1-RELEASE/usr/local/...
[1] https://freebsdfoundation.org/project/import-mit-kerberos-in...
I don't think that has anything to do with FreeBSD's choice of MIT Kerberos or Heimdal.
Well, except the FreeBSD Foundation explicitly says MIT was chosen for interoperability.
Are you disputing the FreeBSD Foundation document?
Er, sorry, I meant the whole thing about Heimdal being non-U.S. based.