The Ambassador Bridge was always privately owned, but when it was first built to celebrate the end of World War I, it at least wasn't under the control of a single billionaire. But things went bad for the company when the Depression hit, so they issued a whole lot of new stock to stay afloat. Half a century later, transportation heir Manuel Moroun bought all the shares that were available, bought a bunch that weren't available, and boom, he now owned a 1.5-mile bridge. Canada tried to step in and get a stake in it, but they failed, despite taking Moroun to court over alleged mob ties.
So, what's the problem with one dude owning such an important border crossing? For starters, bridges have to be maintained, often at a loss, so they don't always work so great as private ventures. When Canada finally gave up on their years of fighting Moroun, they made him agree to make improvements to the bridge, which he promptly did not do. He does still manage to make vast amounts of money off the bridge, though. He gets this by charging his own tolls (money that would otherwise support both the bridge and the entire US highway system) and by setting up his own duty-free shops close to the crossing.
Tired of this madness but unable to wrestle the Ambassador away from Moroun, the US and Canada finally agreed to make their own competing crossing, which they swear will be much cooler than the old lame bridge. It'll be called the Gordie Howe International Bridge, named after a Canadian hockey player who played for Detroit, as he is the ultimate symbol of international unity. Moroun has fought to keep the new bridge from being built, but by the time it's finished in 2024 or later, he'd be 97 years old, so maybe he won't care anymore.