I could think of quite a few reasons, and possibly originally it was intended to make it possible but now the story narrowed down a bit (some stuff wouldn't work without a special connection to Beth I think, or at the very least be even more implausible than it already is).
And I can sympathize, I stopped a different game (to have no discussions about the merits of any individual character) where you can turn down (almost?) everybody, however those who I picked didn't seem to have gotten the message at all and dialogue is wonky as well. I wouldn't even have minded too much if they were enforced in the first place, but getting a choice, making it, and then ignoring it is - except for rare circumstances where it becomes immediately clear that you'll have to go a certain route - offputting and distracting.
Since I didn't reject Beth here I cannot say though how it is handled and whether it even may be a bug and not the "canon occurence".