Do you mean pillerboxed rather than letterboxed?
I am not familiar with the term "pillerboxed" ... I thought it was "letterboxed", but English is not my native language, so I guess I was wrong.
There are black bars on both sides during the opening credits, and they are moving aside when the Enterprise enters the picture right after the credits. And it doesn't seem like the picture was stretched on the DVDs, there is actually more picture on both sides. I wonder if the BD represents the original theatrical version (which may or may not have been on the VHS or LD edition), while the DVDs have changed the credits to full width: