Based on the way we use the term for some terrestrial ecology, I'm guessing that there isn't a really solid reason. Forest can mean a lot of different things, including an area with trees on it, an area with trees on it that is not a wetland, or an area that is usually used to grow trees, regardless of whether or not there are currently trees on it.

I've also heard this called a "seagrass meadow," or "seagrass bed."