I noticed also that no one's sitting in them... they don't look to be the most ergonomically-designed chairs I've ever seen.
I haven't done a full top-to-bottom search of every episode (since I've got bigger fish to fry), but we see this odd seat and chair back modification to a Burke chair base a few times:
In "Taste of Armageddon" (and yes, Kirk is sitting in it):
In "Devil In The Dark:"
And, with a new paint job, in "Operation--Annihilate!:"