Because they apparently require foreign banks to report it.
Now, how do they actually enforce that supposed "obligation"? I do not know.
But it doesn’t surprise me. The US has already enforced it’s laws on foreign companies' dealing abroad merely because they had used dollars in their transactions.
They haven’t enforced it yet (for the banks, for non-compliant Americans, they have). But they’ve signed treaties with many governments to allow this, and they threaten foreign banks with fines related to any US dealings.
Now, how do they actually enforce that supposed "obligation"? I do not know.
But it doesn’t surprise me. The US has already enforced it’s laws on foreign companies' dealing abroad merely because they had used dollars in their transactions.