skaus2001,
In addition to your full-time job, you CAN take up additional jobs IF you are on EAD! On H-1B, you can not, because it is tied to one specific company. But, EAD is a general employment authorization which allows you to take up as many number of jobs as possible.
Consider the case of a dependent spouse. He/She will be seen starting a job once the EAD has been received. And, nothing stops that person from taking one, two or N jobs because their EAD is not tied to a specific company.