You have misunderstood what the legislation is. It is a labor impressment act, only addressing the issue of labor. Prior to this legislation the Confederate government had to contract the use of slaves directly from their owners, as it was not legal to impress someone's slaves into government service for the uses described. The owners could retrieve their slaves practically at any time, which they often did when they were needed again to do work at the owner's property. This meant the government did not have a constant or reliable supply of labor, and was continuously trying to contract labor to not only meet the original quotas, but make up for the ones that had been recalled by their owners as well. The need for adequate labor resources to build defenses and fortifications, etc., was a critical issue, and this act gave the government the authority to directly impress slaves in order to meet their labor requirements instead of contracting out slaves as had been done before. It is only a labor act dealing with labor issues.
One third of the confederate army was impressed, most of the rest were on extended enlistments. So if they are impressed labor only then why the uniforms? Why specify what they may do and, by implication, keep them from doing other, combat related actions. And why the need a year later, to pass legislation authorizing the enlistment of 200,000 more? The fact of the matter was that any blacks with the army prior to this legislation were there in an unofficial, unauthorized capacity, and that they filled solely support roles.