Disney tracks the refillable mugs with RFID tags because the refills are at no charge.
The popcorn bucket refills are a different story. You still pay for the refills, just at a discounted rate. Disney is still clearly making quite a significant profit on each and every refill.
As has already been pointed out, if Disney thought they were losing money, or even a lot of potential profit, on this deal, they'd have put RFID tags on the buckets. They haven't, which suggests to me that they think leaving it up to the interpretation of the individual cast member is working well enough for them.
