Facebook can't set arbitrary request headers easily. That would require messing with how the user-agent retrieves things. Cookies are the chosen way servers can set client request headers. So the tracking-information is passed by a cookie. Now since browsers are starting to be selective about cookies, Facebook becomes inventive.

Adding a request parameter absolutely will break things. And they knew this. The only question was what's worse: Not being able to track some people or breaking some of their links. Facebook decided the former is more important to them and their customers.

And even if nothing else brakes, uglifying the URL people are posting is in itself an anti-feature.