Evidence for Truism 8

Consider the following phrases (the last col say if the phrase is ok, not ok, or awkward.

she cooked	and	it was good	ok
she cooked	and	it was bad	faintly awkward
she cooked	but	it was good	not ok
she cooked	but	it was bad	ok
she cooked carefully	and	it was good	ok
she cooked carefully	and	it was bad	not ok
she cooked carefully	but	it was good	not ok
she cooked carefully	but	it was bad	ok

The explanation is that the first 4 rows are about an event. By default we assume the event went well (Truism 8A) but the assumption is implicit. So row 1 is ok and row 2 is awkward. It contradicts something only weakly assumed. But the last 4 rows all are about the quality of the event explicitly. So the "and it was bad" and "but it was good" don't work at all.
Truism 8B applies to the last 4 rows and makes it implicit that the result is good. This aligns perfectly with the last column: you say "and" and expect good, and say "but" and expect bad.
Update: The difference between "awkward" and "not ok" is that row 2 embodies two assumptions (both 8A and 8B) while row 6 embodies only one of the truisms. It makes sense that one implicit assumption generates more expectation than two do.