What is the motivation behind Key-policy Attribute-based Encryption?
I believe the answer cygnusv gave is not fully correct. If an object is tagged with "NUCLEAR, TOPSECRET" it can potentially be decrypted by someone not having the TOPSECRET attribute (or NUCLEAR attribute). Why? Because it all depends on the structure of the private keys in the system. 

A private key could for example be: "NUCLEAR or SCIENCE LAB A". Thus it would suffice for the object to be tagged with NUCLEAR for this person to gain access.

This is also one of the reasons to why KP-ABE is "less intuitive" because the encryptor does not specify the relation between the attributes that are tagged. Instead it is the key authority (or just authority) which will decide how these attributes needs to be combined for access.