Mikaela broke the Entity's own rules

This would imply the entity has a greater sense of self and morality. "Dark" and "Light" magic are more an extent of moral nuance, the idea that certain magic carries an aura of evil or of purity. Comparatively the entity is more like a semi-sentient force of nature. Cheryl can channel it despite being a "good guy" because the entity is both a life form and a form of usable power. We've gotten further examples of the entities usable power from characters like talbot, who effectively created a super soldier serum out of blight secretions. The entity allows this "light magic" because it is a further extension of its own will and power. Totems are a direct link to the entity, killers call upon the entity to hex the totems and grant them its influence. Mikaela does much the same, blessing a totem to call upon the entities influence in favor of the survivors. It has no greater sense of morality, of good or evil, it simply follows whatever path feeds it the most. All other considerations are utterly secondary to it. This is why survivors with supernatural links (Cheryl, Mikaela) can utilize its power, to an extent. If it did not desire mikaela to perform such magic, or felt threatened by it, she would be taking a swift vacation to the void.