It seems to me that the 'thumb' of the seam ripper would leave a trail of displaced molecules behind it when making the cut. Perhaps this effect is negligible if you cut down the column rather than up.

I think what I would do is freeze the straw solid and either slit the straw lightly with razor blade or use a dowel to push the column out the end.

Side note: for a thicker column, there are these straws used with 'bubble teas' in chinese restaurants. Presumably they are still relatively cheap, albeit a bit more difficult to find than grocery store straws