How They Got the Second Suspect

Well. The accused has rights, but those rights do not include "being read his rights". They include things like "the right to remain silent" and to a defense attorney. The accused had rights before they invented the Miranda Warning, and said warning's main value is as a procedural guard to make sure that no one's rights were violated because the accused were not aware of their rights.

What he can do when he's tried, and when they're using any of his testimony against him, is that he can say "I was not read my rights" and then the judge can ask "were you aware of your rights?" and if the court determines that he might actually have been unaware of his rights then they can throw that testimony out, or provide other remedies.

I have my suspicions that his own testimony won't be the deciding factor in any criminal proceedings, however, so it may come to nothing in the end.

(No commentary on the appropriateness or inappropriateness of the decision to not-read-him-his-rights is herein advanced by this comment, except that it would have at least avoided the fuss which is currently being made about it, so that is a point against it.)

Carmen Ortiz, arguably the person who drove Aaron Swartz to his death, is the same person who made the decision not to read Boston bombing suspect Dzhokhar Tsarnaev his Miranda rights. Apparently the rights of the accused aren't a high priority with her.

“public exemption in cases of national security.”

This is a huge stretch for the definition of "national security" and not necessarily one that a US Attorney should be making in this case, lest courts rule against them for over-extension and they lose the privilege of invoking this exemption for the future.

If Tsarnaev will get a good lawyer - this will play well in his defense in court after emotions will cool down.