After looking into this, I'm wrong and I'm going to edit my original comment. My apologies.
Where I'm confused is the difference between a direct line of ascent (even when there is an overhead directly above) versus penetrating beyond that. This is a differentiator that makes swim-throughs OK but most caverns not OK.
Where I'm confused is the difference between a direct line of ascent (even when there is an overhead directly above) versus penetrating beyond that. This is a differentiator that makes swim-throughs OK but most caverns not OK.
Edit: Turns out I can't edit a comment that old.