I've only eve ridden solo and to me it would not even occur to me to draft another rider. If he's riding slower than I am I would pass him, perhaps wave hi. If he's the same pace as I am I would likely drop back a good distance initially, but if I find he's slower down, I would just pass him. He is of no consequence to me.

Not everyone who rides on public roads are familiar with general 'guidelines of conduct' for cycling. Perhaps just general etiquette and decorum should apply. If I get on a near-empty subway car I'm not going to sit right next the the only rider on it. Another rider tailing me at two-bike lengths would be a very unwelcome sight, and yes, I do, on occasion go 20+mph, on flats.

In any case, if you were out for a solo ride, you're expecting to be 100% your own effort. If you wanted to draft you should ride with a partner or a group.