[sumgocko]

(This is baseless speculation by someone who has only taken Genetics 101)

Human cell nuclei produce mRNA from DNA, which then makes it's way to the ribosome to be expressed as a chain of amino acids, or a protien. On it's way to the ribosome, it needs protection from the cell's RNA cleanup enzymes, so it leaves the nucleus with a 5' cap and a poly-A tail.

Therefore, it makes sense that a virus that wishes to hijack human ribosomes would end in a poly-A tail (a bunch of As) in order to not get destroyed by human cells.