Pokemon like Latias can use Roost, but take no damage from ground type attacks, even though it makes the Pokemon land.
Replay for proof.
Is this a glitch, or meant to be?
Roost restores half of the user's maximum HP. If the user is a Flying-type, it will lose its Flying-type (thus being treated as a single-type Pokémon) until the next turn. If a non-Flying-type uses Roost, then there are no side-effects.
So, yeah its not a glitch. Non flying type Pokemon do not become grounded or lose their "flying type" because they dont have one to begin with. This will only happen with actual flying type Pokemon.
Source: http://bulbapedia.bulbagarden.net/wiki/Roost
