Why doesn't the PLA let people take pictures?

If you mean the barracks, docks, airports and equipment of the army, it really belongs to the military restricted zone or the military administrative zone, so you can't take pictures.

People's Republic of China (PRC) Military Facilities Protection Law

Fifteenth people, vehicles and ships outside the land and water management units in the military restricted zone are prohibited from entering the military restricted zone. It is forbidden to take photos, video, record, survey, measure, describe and describe the military restricted zone, and it is forbidden for aircraft to fly at low altitude over the military restricted zone. However, unless approved by military organs at or above the military region level.