So would I. But what would be your damages? Assuming you did not contract an illness, perhaps $1 actual plus punitives, and the Supreme Court has declared (in a recent, infamous opinion protecting an insurance company from its own oppressive conduct) that it is unconstitutional for punitive damages to be more than 5x actual damages. So spit away, all you have to fear is possible criminal prosecution.

P.S. After writing the above I found a civil case from the U.S. Court of Appeals for the Seventh Circuit in which an award of 37.2 times compensatory damages was upheld [pdf] against Motel 6 for knowingly renting bedbug-infested rooms. There is a discussion of the possibility of punitive damages for spitting....