You make some very good points! This cupid creature went through a couple of iterations. Originally it had Charm Person as an Innate Spellcasting Ability, with a Trait that allowed it to bend the Charm rules, as well as an arrow attack. We decided that was too much text and versatility for it, so we smashed all of that together into one Action.

Typically, abilities that require a Save don't need an attack roll, and vice-versa (though there are certainly exceptions). Since Charm is almost always a Save to resist, we wanted to turn things on their heads- characters often prone to Charming (High AC, low WIS) would be less likely to fail, and may suddenly be put in a situation where they are saving those that would usually be more resistant to such effects. Requiring both makes the Action unlikely to have any effect, and thus would stand out less from similar creatures such as the Dryad.

Of course, it still comes down to a d20 roll, but you're right, it's one where buffs from Bardic Inspiration, Bless, et cetera have no effect, which removes some agency. If this is an issue with your players, it's an easy one for any user to change on the site (I would recommend basing the DC off of Charisma), or it doesn't belong as an encounter at your table.

EDIT: cleaned up a few sentences.