Since XFA forms can be dynamic, the subforms can potentially grow outside of the current page.
This can only happen if the Parent subform:
1. Can reflow content (static subforms don't grow)
2. Can also allow page breaks.
So these are the conditions under which you can have a page break within a subform. Notice that this option is enabled it is defaulted to checked. It's provided so that you can block page breaks, not add them.