I'll start off by saying that I'm not sure if this is a bug or a feature necessarily, but I was unable to find any Civ/Leader trait that would provide this, but one of the Civilizations in a game I started a few days ago has full patronage with all Independent Cities regardless of proximity.  This particular Civ does not appear to have a large naval presence (I'm the pre-eminent explorer in this game), and it even impacts land-locked Independent Cities.

I'm not saying it's impossible, but I feel it has to be highly unlikely.  And, again, I haven't see anything that would suggest this Civ is driven for exploration or relations with independent cities above all else.

If this is not a bug, but a feature, then it's a bit unreasonable to not allow the assimilation of independent nations if someone is already patronizing it.  Or provide competing Civs avenues to reduce patronage of the other (espionage, bribery, etc.), instead of outright conquering them.  If I'm incorrect and there is a way to do such a thing (after looking for about a half hour, I couldn't find one), I'd be happy to retract my bug report.

Much appreciated!  I'm enjoying the game so far on average, and look forward to new patches/expansions.