My thought is that Disney doesn't pay for the products so I'm not sure how much credence I put in their refil policy, expecially when they don't enforce it themselves. The coke a cola company puts it's products in Disney for free to promote it's brand. If you spend 12.95 for a refillable mug or buy a paper cup, that's all your paying for, since Disney actually has never paid for the liquid. I figure they can sell you a cup but they actually don't have a say in how you fill it since they don't own the product. They would prefer that you buy new cups every time you visit since it's almost pure profit, but since they pay nothing for it, they never say anything about people refilling old cups because they're not losing anything THEY DON'T CARE. If a Coke representative wants to say you can't refil certain cups then they have that right.
I did not buy a refillable, I bought two cokes while there and I paid for both of the paper cups they came in. However if I did buy a refillable cup I would use that cup as long as they were current with out any guilt. I would not consider it stealing nor would I consider it breaking the rules. Now that the new cups are out, if I had a blue YOMD cup I would consider it not current, and that is where I personally draw the line in the coke product debate.
But as far as I'm concerned anyone with a mug from any year is not doing anything wrong. Filling up non Disney purchased containers, that would be wrong. But only because I think it's something a trashy person would do. But that's just me.