I'm not sure it's about the amount of plastic used, it's about being able to recycle or reuse it properly. It's really not that hard to recycle things at home or in public, just throw it in the bin. I understand the concept that individuals have very little impact but I'd rather be a part of the solution than part of the problem, if that makes sense.
As far as reusing, I don't see what's confusing. Talking on a more personal level, if I buy an iced drink or something at Starbucks, for example, once the drink is empty I often rinse the cup and refill it with some water or something after I get home. It's so I'm not dirtying another glass and I quite like the size and durability of their cups. I have a perfectly good drink container and just because it once held iced tea or something doesn't meant I can't use it for some water and then recycle it later. The straw is kind of an issue when mixing different types of drinks. So shifting to a cup that can be rinsed and dried has potential to be more reuse friendly would be nice. And hey it might even encourage people to reuse a bit more before immediately thinking to throw things away.