When a client voluntarily makes a request to a server, it presents a bunch of information for the server to see and consume. This information is not meant to be kept secret from the server. Among such pieces of information can be some about the characteristics of the user agent, including OS. It is disingenuous at best to call collecting such voluntarily-presented and clearly-transmitted data as "spying" on a user.

A basic requirement for spying is for a collecting party to be obtaining information that can be reasonably considered confidential or restricted. Details about the system from which you send a request are by definition of the protocol not confidential or restricted to the recipient of your request. It is not reasonable to expect a server to not look at or use information you present to it. Therefore, it isn't "spying" for the recipient to consume the information. The information might be used in ways some people(e.g., OP) don't like, but that does not make obtaining the information "spying".