Where on earth do you get an idea like that?
No, the US is not the only democratic nation with uncapped campaign spending, for any plausible definition of "democratic" and "campaign spending".
The caps in other countries might not implemented in an umbrella dollar amount but as rules like no TV ads, or the dollar amount could only apply to certain items. Often it is a cap on donations which effectively caps spending. At the end of the day, democracies generally must limit campaigning influence because if they don’t, they might end up with skewed democratic process, possibly a reality show star as president.
This is a gross misunderstanding of the Citizens United ruling.