Why do people think John Zorn's music is jazz?

Some of it is, most of it ain't.

The Sonny Stitt and Ornette Coleman tribute albums are definitely the "jazziest" in a way.
There's lots of jazz on the Masada records (acoustic and electric).

Zorn has released so many records that it's almost impossible to keep track of all of his projects.
One could say that many have an affinity to improvised music. Some would call that jazz too.