Bush and free market principles

"I readily concede I chucked aside my free-market principles when I was told ... the situation we were facing could be worse than the Great Depression," -GWB (link)

So, if it's a good idea to chuck them aside in the face of economic disaster, what's the argument for retaining them we don't face economic disaster? Or, more generally, what's the rule specifying the economic conditions in which free market principles should be used and those in which they shouldn't?