Why does this measure ignore the slicer?

A user reports that this measure shows the same value no matter what they select in the Region slicer.

dax
Total Sales UK =
CALCULATE (
    SUM ( Sales[Amount] ),
    Sales[Country] = "UK"
)

Wait — that one is expected to be fixed to the UK. Here is the actual problem measure:

dax
High Value Sales =
CALCULATE (
    SUM ( Sales[Amount] ),
    FILTER ( ALL ( Sales ), Sales[Amount] > 1000 )
)

Why does this measure ignore the Region slicer, and how would you fix it?