This is true of other powers as well.
America has a "mutual defence treaty" with the Philippines, yet they did not stop China from seizing the Scarborough shoal in 2012. They abandoned them.
America also promised to protect Ukraine's borders in 1994, in exchange for Ukraine giving up their nukes. But that did not stop Russia from annexing Crimea in 2014.
Even North Korea can launch multiple lethal attacks on South Korea, one of America's closest allies, without repercussions. They even sunk a South Korean warship, killing hundreds of naval personnel, and nothing happened.
In geopolitics, national interests reign supreme. No American was willing to risk a nuclear war over Crimea, regardless of their previous promises and treaty obligations.