The purity of angels comes from their obedience to God. Like any other rational being, they had the opportunity to disobey Him. Some of them did, but some of them kept themselves under the light of obedience to God. Obedience requires power, wisdom and liberty. It is not slavery. It is exactly the opposite.
In this photo a monk from Vatopaidi while patiently standing at attention with his katsion at the end of a long festal service.