Personally, I believe the fault lies with the cm's who said you could keep bringing your mug back. This May, I encountered the person in front of me in line being told just that (I had just bought five new mugs, and was surprised when this cm actually said they were good at any resort..first I had heard that!). Some suits were at POR, measuring and discussing with workers at the food court station a change (this was in May) that was going to take place (at the time, he said they were upgrading it to be more like the Allstars, with better drainage, garbage cans along the drainage etc), and I asked about the policy, and he said the cm was incorrect, and they hoped to be "fixing" that wrong info.
As far as the sign, I believe the thread in the budget board has a picture of it. I've seen the sign at all the Allstars and POR. It does clearly say it is for length of stay. At Allstars the sign is on the wall by the exit nearest the pool..and also as you go in, on the right (where they do the charactures at night) wall that tells you what is available. It's pretty large.
I don't think anyone is cheap, or trying to get away with anything. That has been the "unwritten " policy at WDW, and many have been told time and again, it was ok...and although I use my FP's at the times I'm suppose to..again, if the policy had been stated over and over by cm's that I could come at a later time, I would expect that that is OK. Disney either needs to train their cm's better, or admit they just don't care if you bring your mug back. Most people don't read the signs, obviously (I didn't until I read here it was up), and we just take the cm's word. Like others, I have asked managers in the foodcourt (beliveing they would know their own policy), and they too, say, sure, bring it on back (I hadn't thought I needed to email Disney about the official policy). So maybe we ought to lighten up on the people who were told it's ok...and wait and see if Disney finds a way to correct this mess. I personally would rebuy (as I did this year when we went to POR..although at that time, I expected to be able to bring 'em back), since I think it's such a deal.