From the article: "One practice that introduces bias into the shaping of defense policy is the revolving door between the U.S. government and the weapons industry. The movement of retired senior officials from the Pentagon and the military services into the arms industry is a longstanding practice that raises serious questions about the appearance and reality of conflicts of interest. Mostly because employing well-connected ex-military officers can give weapons makers enormous, unwarranted influence over the process of determining the size and shape of the Pentagon budget."

This seems to answer your confusion about why they are concerned about this.

The article also seems to suggest that they aren't trying to ban former Generals from getting jobs, they would just like to regulate it better, the first two proposals seeming to be pretty important:

- Bar four-star officers from going to work for companies that receive $1 billion or more in contracts with the Pentagon annually.

- Extend “cooling off periods” before retired officials can go to work for the arms industry to four years. This would ensure that key contacts or key information that the official may have been privy to while serving would not provide an outsized advantage.