Nor should any man buy a woman tampons. Even if she's laid up with an injury or a flu, really...come on...it's not like it's a surprise that sprung up at the last minute unexpectedly; it comes every godddamned month around the same time, and has done so ever since the first one. There's no excuse for not being prepared with proper supplies.
You'd be surprised at how many women have very irregular cycles. Everyone isn't on a 28 day routine.
But either way, once you've been together a while, buying tampons or whatever for your SO shouldn't be/won't be an issue. Sure, most all single guys say "eww no way, no how" but reality is, things change over the years.