Erin Bromage, an associate professor of biology at the University of Massachusetts Dartmouth, explained that the masks are designated as "single use" because they are typically categorized as medical masks, so health care workers would need to swap them out more frequently to avoid "cross-contaminating a patient room with equipment that was worn in a room of an infectious person and then moving to the next room and bringing that infection with you."
"When you then take a medical-grade thing that's single-use and put it in the general public, we're not worried about you cross-contaminating different environments you're being in. It's really about providing protection to you." Full story from iHeartmedia linked here