You already know the answer to this. Fannie and Freddie sought to buy risky mortgages, ostensibly to make it easier for people to get credit.
Edit: As if the above isn't common knowledge, here's a link to Fannie bragging about their "chartered mission to increase the amount of funds available in order to make homeownership and rental housing more available and affordable": http://www.fanniemae.com/kb/index?page=home&c=aboutus
Here's a link to Fannie easing their underwriting standards: http://query.nytimes.com/gst/fullpag...5AC0A96F958260
They were classified as grade-A because their value was ensured by the demand of government sponsored enterprises.
So the mistake that private companies made was to go along with that government scheme. And your conclusion is to blame private companies for their participation instead of the politicians who devised the scheme?
And you are well aware that mortgages meeting private lending standards were not a part of the problem.