It's a euphemism for something that does not deserve to be brushed off and belittled. Not every fan of a series wants female characters treated this way and many people that may be interested in a series are pushed away as a result of this treatment. It seems to excuse developers as "just giving fans what they want" and not holding them accountable for their sexism.
What are your reasons for using the phrase "fan service" instead of objectification or sexualization?
What are your reasons for using the phrase "fan service" instead of objectification or sexualization?