Here's how it works: In a cs map, such as office or italy, the T team is allowed to camp, since their objective is to stop the CT team from rescuing the hostages. But, if a CT does get the hostages, and the T team does nothing to stop it, they are camping.
In a de map, such as dust, the CT team is allowed to camp because their objective is to prevent the T team from planting the bomb. But, just as Zimmy stated, if the bomb is at B, for example, the CT's can't be camping at A.
The More You Know...