People who call others autistic for being interested in anything at all are usually just projecting autists themselves.

If your social life is fine, you can easily pick up on social cues, and you are capable of making friends normally, you are probably not autistic. Autists don't own the concept of obsessions, as this is something that shows up in multitude of disorders. "On the spectrum" is just a way for them to turn it into semantics, or to ignore your arguments entirely.

As to why they do this, I'm not sure. Maybe it stems from an insecurity. If they see everyone else as autistic too, they no longer have to see themselves as inferior.