In which case I suggest you inform Merriam-Webster and the Oxford English Dictionary as they are both under the (Ahem) mistaken impression that it is a plural noun.
What kind of activity does "I'm going to hospital" imply that I'm participating in?
Just as going to church implies that you are going for spiritual renewal going to hospital implies you are going for some serious physical renewal.
Then why don't we study "biologies"? Or "chemistries"? Mathematics is a subject, as are chemistry and biology. They all occupy the same place in the language. (And wouldn't it sound funny to say "Mathematics are a subject"?)
World English Dictionary mathematics (ˌmæθəˈmætɪks, ˌmæθˈmæt-)
n 1. (functioning as singular) a group of related sciences, including algebra, geometry, and calculus, concerned with the study of number, quantity, shape, and space and their interrelationships by using a specialized notation
2. (functioning as singular or plural) mathematical operations and processes involved in the solution of a problem or study of some scientific field
[C14: mathematik (n), via Latin from Greek (adj), from mathēma a science, mathēmatikos (adj); related to manthanein to learn]
Note that the original word in Greek has an "s" ending.