You all know that report parameters can be populated through a query, right?  And multi-value parameters get this magical (Select All) item as first one in the dropdown list.  I used the word magical in the previous sentence because this (Select All) item is not a real one.  It’s a bit like a fata morgana: you can click it but it doesn’t really get selected.  It only appears that way in the dropdown.  But you can’t write an expression that tests if the (Select All) item is selected because it doesn’t really exist in the array.  The only thing it does is it selects all items when it gets clicked.