Why is Bubble Tea a cuisine and Ice Cream is an amenity?

ice_cream exists as a cuisine value too, see https://taginfo.openstreetmap.org/tags/cuisine=ice_cream and you can simply add it as a further cuisine value if necessary.

That there is a dedicated amenity value for a place that sells ice cream over the counter is mainly historic (the bubble_tea value was introduced in 2015, amenity=ice_cream pre-2010). Sure one could argue that a more consistent tagging would be amenity=fast_food + cuisine=ice_cream but there is no harm in using the dedicated tag.