The key difference being that it is perfectly legal for the US to request data on income and gains received by US taxpayers while it is illegal for the US to spy (in certain ways) on US residents.

It is completely routine for countries to exchange data on financial accounts [1]. The only aspect that makes FATCA somewhat unusual is that the US taxes US persons even when they are residents of other countries.

[1] https://www.gov.uk/hmrc-internal-manuals/international-excha...