It's mostly regulated to fully-evolved Fire types, and this near-universal compatibility with fully-evolved Fire types and the Solar Beam TM wasn't enacted until gen IV.
It's especially odd since there are numerous examples of Eeveelutions learning damaging moves of their relative's types over the years:
-Vaporeon: Aurora Beam, Bite, Blizzard, Ice Beam, Icy Wind, Stored Power, Synchronoise, Trailblaze
-Jolteon: Bite, Stored Power, Synchronoise, Trailblaze
-Flareon: Bite, Stored Power, Synchronoise, Trailblaze, Zap Cannon
-Espeon: Bite, Dazzling Gleam, Draining Kiss, Grass Knot, Magical Leaf, Trailblaze, Zap Cannon
-Umbreon: Dream Eater, Psychic, Stored Power, Synchronoise, Trailblaze, Zap Cannon
-Leafeon: Bite, Knock Off, Stored Power, Synchronoise
-Glaceon: Aqua Tail, Bite, Chilling Water, Mirror Coat, Stored Power, Synchronoise, Trailblaze, Water Pulse
-Sylveon: Bite, Magical Leaf, Mystical Fire, Psychic, Psyshock, Stored Power, Synchronoise, Trailblaze