Yeah, but the title as presented in the film was just plain Superman, with no subtitle. I'd be curious to know why it was marketed as Superman: The Movie.
Presumably the same reason as Twilight Zone: The Movie, Transformers: The Movie
, GI Joe: The Movie, Mighty Morphin Power Rangers: The Movie
, and Mystery Science Theater 3000: The Movie
. Plus The Muppet Movie, The Simpsons Movie, The Spongebob Squarepants Movie, The Brady Bunch Movie
If something originated in another medium, it makes sense to call attention to the fact that this is a movie, something distinct. Not only because of the "event" factor, but because not all promotion and discussion of the film will be in movie posters or theater marquees. Some of it will be word of mouth, tie-in ads, etc. So it's worthwhile to point out that it's a movie and not a comic book or TV show or whatever.
It goes the other way too. RoboCop: The Series, War of the Worlds: The Series, Friday the 13th: The Series, Men in Black: The Series, Godzilla: The Series, Highlander: The Animated Series, Batman: The Animated Series, Blade: The Series