At this point, I don't think WDW is losing money on the mugs. As someone mentioned, people will spend the money saved on something else instead. And I do think it is a wonderful idea, although I don't utilize it myself. I'm just concerned that the day will come when, due to people's different interpretation of the "rule" (including CMs) it will become too costly, and will be discontinued.
Prior to the internet, there weren't enough people aware of some of the "loopholes" (no matter how legal) in the "rules" to have much of an affect on WDW profits. This is changing more and more as people figure out ways to get around rules without actually breaking them, or without being caught, and they share that information with hundreds of others. (The old "If they really, really meant for you to follow a rule they would do a better job of enforcing it, and would make sure that NO CM ever made an exception, and ALL CMs would have to be aware of ALL rules in their area." argument )
Again, I am not criticizing anyone who ASKS a WDW employee a specific question about something they are not sure is allowed, or for clarification of a certain policy, (such as, can I use these mugs on my next trip?)and they act based upon that information. I'm criticizing the people who take the "Well, if so and so did it, and no one said anything, it must be OK for me to do it, too!" attitude, or the "Anything not specifically prohibited must be allowed." line of reasoning.