In fact, you usually can just send arbitrary query string parameters to a server - that's why the behavior is so common, and often useful.

Most sites don't mind or break, some sites get value from the behavior in ways hard to replicate in other ways – and those sites that don't like such additions can easily ignore them. And a few lines of code will work better than ineffectually appealing to manners, when the freedom of the web's form of hypertext, and protocols, gives the outlink authors full freedom to craft URLs (and thus requests) however they like.

Crafting outbound links with your own additions and handing them out to visitors to your site is similar to the practice of writing someone’s phone number on the door of a bathroom cubicle with ‘for a good time call:’ written above it.

You’re handing out someone elses’s contact details, but giving the person you hand them to a completely fabricated expectation for how the interaction will go.