According to news reports, Obama wants to prevent the ousted executives at Fannie/Freddie from collecting on their severance packages. Although they are likely to walk away with large packages per their contract, there doesn't appear to be evidence of illegal behavior by them. Absent such evidence, the government doesn't have the power to abrogate their employment contracts.
Having said that, I expect we will learn they are walking away with millions of dollars. While CEO pay is often obscene, it is not yet illegal and Obama knows it.