Samsung is "exempt" because they are beholden (in theory) to a government that shares American values (in theory).
It's notable that Director Wray's response included language about "foreign governments that don't share our values":
> "We're deeply concerned about risks of allowing any company or entity that is beholden to foreign governments that don't share our values to gain positions of power inside our telecommunciations networks."
Their military is also quite intertwined with ours -- the US maintains operational control in the event that S. Korea goes to war, at least for a few more years.
It's notable that Director Wray's response included language about "foreign governments that don't share our values":
> "We're deeply concerned about risks of allowing any company or entity that is beholden to foreign governments that don't share our values to gain positions of power inside our telecommunciations networks."