I'm here at Disney now..and chatted with one of the the food court managers at length. Told him of all your concerns..he was surprised to find so many are more concerned than they are. He says it's prob less than one in a thousand bringing back mugs (and he thinks that is a very high estimate), and he has no problem with it. He says he looks at it as good cutomer service, and in some instances, that might be just the thing to bring someone back to a Disney hotel (which we're told down here are just not filling), if they think Disney people care about their dollar.
I looked around both times I was down there..and I seemed to be the only one this trip refilling an older mug. One person filling a park mug, but the rest all have the newer mugs.
And did I hear any of the cashiers telling people to bring them back..no, but...well, the manager did, as I stood talking to him..a family was considering purchasing, and he told them they could bring them back on their next trip.