Basically, the argument is that, by increasing the number of labs that handle bioweapons, you increase the risk of a bioweapon either being used or released by error.

“The simplest, most likely path for an individual or group to acquire a bioweapons capability is to obtain bioweapons agents and training by penetration of a U.S. bioweapons-agents research project,” said Richard H. Ebright, a scientist at the Waksman Institute of Microbiology at Rutgers University in New Jersey. “One well-placed graduate student, post-doctoral fellow or technician... It’s only a matter of time.”