It is specifically "US hegemony" and not "western democracy hegemony" because the US is so extraordinarily powerful in economy and military.

Interests/culture with other democracies aligns well enough (and the power differential is large enough!) that US leadership is tolerated/supported.

But Canada, EU, Australia, Japan are NOT vasall states: If interests would clash and/or the US lose a lot of its relative power, those would cease being majority supporters and push for domestic interests instead.

Calling them "fairweather friends" might be too cynical but I think it's much more accurate than considering them integral parts of the US hegemony.