Did you read the rest? Obviously its not a good thing, but it is a very efficient way to relieve heat from the PSU. An exhaust fan on top of the PSU puts the air out through the top, where it is quickly put out of the case by the rear and top case exhaust fans.
A vent in the front of the PSU using a front fan doesn't work for a few reasons. The air movement from the smaller rear PSU fan doesn't exhaust the hot air well enough, and the front case fan is far enough away to not provide a great amount of cool air into the PSU vent. That hot air tends to stick around more because of lower CFM.