Reddit is owned by a mass media conglomerate. A significant portion of everything on the site is native advertising. Self promotion would interfere with what they are really pushing and undermine their influence. The smaller communities exist as goodwill to make the high traffic portions seem more authentic.

Using sponsored headlines makes you look even worse than posting unfortunately. You have to be an insider to get the native placement I guess.

Just watch for the next big hollywood movie to come out and random disguised content x to magically bubble up and plaster the front page.