First, I would just like to say I do not believe what I am about to say... but someone needs to argue the other side.
Rich people are more valuable to society than the poor. Rich people have money, and it is their use of that money which provides economic activity, investment, business and employment. The homeless man provides nothing to society, and will consume the same resources he did on the street as he will in the prison, and will create nothing just as he did before being jailed.
Also, white collar crimes are non-violent, while the man committed violent, armed theft (the requirements of robbery, dumbed down, of course). There is a greater cost to society in robbery, even of a small amount, than large amounts of money stolen in a non-violent way. Robbery can lead to much worse crimes, including murder, while white collar crime is not similarly likely to do so.
Finally, there are greater safeguards against white collar crime already in place than safeguards against robbery. White collar criminals are already deterred by economic regulations, investor protections and increased transparency in the economic system. The victims of white collar crime are often guilty of allowing the criminals to get away with their theft because the victims either believe the dishonesty will make them rich, or because they are careless with their money. There are no similar safeguards against robbery, especially with police enforcement dwindling due to budget cuts.
Future robbers are not easily deterred from committing their crimes. For this reason, harsh penalties must be common. White collar criminals have much more to lose just by virtue of having been convicted, thus they are more easily deterred by smaller sentences. Since the deterrence will not increase much due to the already high cost of short incarcerations, the cost to society of imprisoning the white collar criminal is unjustifiable.
OK, time to go wash myself off... but at the same time, you know that some of the above is correct, at least in the eyes of the US judiciary.