I've personally been on support screen sharing conferences with the DoD before as a third-party consultant/contractor. They do not provide all contractors, especially third party, with DoD managed devices and in those cases I always thought that it was a bad practice. I asked for the people on the conference to e-mail me the root CA cert to validate the thumbprint was the same as the site but I'm not sure everyone would do that and instead blindly choose the 'proceed anyways' option.

/edit. That was a very long time ago though so I'm not sure if they're even using that same screen sharing site anymore or if they've since changed it to use a public CA root cert.