The J-1 visa is for training. The agencies typically want to see that the foreign national is coming to the U.S. to obtain training in some designated field and will use that training to further his/her career upon return to his/her home country. This is not a method for companies to hire workers, but rather to train them. The entire duration of the J-1 visa is meant to be for hand-on learning. For example, if your training plan shows you will be training as a manager in a restaurant but instead you are waiting tables, this is a violation of the J-1 visa.
On another note, as a J-1 visa holder you receive "stipends" not a salary. This is to again differentiate between someone who is there to learn and someone who is there to work.
