I'm not a Disney bean counter but I think the reason would be more closely aligned to that resale/eBay market. If Disney sells a mug for $8.99 for a single day's use, how much of that goes to pay for the mug itself and how much goes to pay for the beverage?
Now, assume that I got a "free" mug with my
free dining this fall. But I have friends/family/co-workers who will be going to Disney in December but not with a dining plan. I could loan/sell/gift my mugs to them if the mugs could be re-used by just paying a few dollars to add more days onto their use. Can you see where Disney would be losing money on the sale of a brand new mug to my friends/family/co-workers? There are enough people who would do this that it could impact the bottom line.
It may not be the most environmentally-friendly thing to do, but this move sets Disney up to maximize their profits on the sale of drink mugs. I really don't think that they care about the few cents worth of soda that people who re-use their mugs take every year. What they care about it how it cuts into the profits that could be realized by selling those people (and their friends/family/co-workers) another mug on each visit.