RIGHTLY enough

It is often said, rightly enough, that America is a nation of laws. Meaning we are governed by laws -- state, local and federal -- that have been properly created and that do not violate the sovereign law, the Constitution of the United States, and the constitutions of the various states.

1. which meaning of "rightly" is proper in the above context? correctly or understandably or certainly?

2. is there a difference between the following 2 variations? different tones?

a. It is often said, rightly enough, that America is a nation of laws.

b. It is often said rightly that America is a nation of laws.

(encarta.msn.com)
1. correctly: correctly, properly, and appropriately
As you quite rightly said, we agreed to this at the last meeting.
2. understandably: with very good reason
She was furious, and quite rightly so!
3. certainly: certainly or positively ( informal )
I don't rightly know.