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The POST body is set to ON; use the data argument:

import requests
headers = {"Content-Type": "text/plain"}
url = 'http://example.com/rest/items/z12'
res = requests.post(url, auth=('demo', 'demo'), data="ON", headers=headers)

The params argument is used for URL query parameters, and by using a dictionary you asked requests to encode that to a form encoding; so ?ON= is added to the URL.

See the curl manpage:

(HTTP) Sends the specified data in a POST request to the HTTP server, in the same way that a browser does when a user has filled in an HTML form and presses the submit button.

and the requests API:

data – (optional) Dictionary, bytes, or file-like object to send in the body of the Request.