What makes you think they're actual servicemembers?
This is Hollyweird. They have actors there, you know. Folks whose business is dressing up in costumes and presenting themselves to the public as something other than what they really are.
Jus' sayin'.
She was apparently at some formal function in the White House being beamed in live to the Oscars. The military people behind her were in various military branches' dress mess uniforms, the highest level of formality, which would be expected for a White House affair.