Who determines what a default position is ?
No who; what. And it's logic that determines the default position.
First off, you certainly can't accept a claim you haven't considered.
Once you have considered it, in the absence of any facts to support one side or the other, you can take one of three approaches:
1. Accept the claim.
2. Not accept the claim (which is different from rejecting it, BTW).
3. Accept some claims but not others.
If you do Option 1 for any serious number of claims, you end up accepting mutually contradictory claims, so that approach doesn't work.
If you do Option 3, you have to divide claims into 2 categories, but without any rational basis to do so (remember: we're talking about a default, so it's before you have any facts at hand). This is logically untenable, too.
This leaves only Option 2, which creates no logical problems at all. It's the basis of skepticism: only accept a claim when you have justification.
If you want something different as the default, please point out where I'm wrong.