I like your definition best (above), but I think to Roberts, who was looking for some way, any way to justify this, the subject matter being taxed is "existence," which makes it a Capitation Tax? I think that's an incorrect view, because you can avoid the poll tax by purchasing something. IANAL, so my opinion on this isn't worth much, admittedly, but it seems to me one has to consider the insurance premiums a Capitation Tax, and that violates all the words in Webster's Dictionary.
Is Roberts a heavy drinker of alcohol in the morning?