Let's just revisit the concept of a "bond".

What is a bond backed by?

It could be backed by an asset, or it could be backed by the expected "cash flow" on something (think of a "toll road" or the "power to tax") or it could be backed by the creditworthiness of a customer (think of the average person on this blog.)

US T-bonds are backed by the "future productivity" of the denizens of the US. Nothing so complicated so far.

Same thing with the Fannie MBS's. They are backed by the assets of the US which in turn are backed by the incomes that will flow through as payments. (I'm doing a bit of simplification here so don't crucify me. I can make it really precise and bore the living crap out of everyone, OK?)

That having been said, they (=China, Japan, etc.) own an awful lot of those MBS's, and they wouldn't like getting stiffed. So the US had to guarantee the bonds.

Now, for all their faults, FNM and FRE were (relatively) conservative so there's a good chance that those bonds are "mostly" going to get paid off. The government is guaranteeing the rest of that "mostly".

This is to basically maintain the creditworthiness of the nation as a whole. Now, you could argue that we could stand to do a bit of belt-tightening but you know how it goes with "cold turkey" and all that.

Does this make more sense?