Not saying I'm actually taking a side. However, as a DVC member you do pay for all costs associated with the DVC resort through your dues, presumably that would include bell services. For DVC and Cash Resort combos the split of cost is done by projected occupancy numbers.
For simplicity sake take SSR or OKW, the dues at those resorts supposedly pay for 100% the cost for running the resort. Where these types of fees get concerning as a DVC member is Disney could be double dipping, charging guests the $6 fee and charging DVC members the full cost of Bell Services at the resort. So Disney then gets the Bell Services cost fully covered by you as a DVC member but pockets the $6 as a pure profit. This is explicitly something not allowed since you can only be charged actual costs, this is why you aren't charged for parking costs. This holds true at the DVC/Cash combos but I simply chose the DVC exclusives for simplicity.
However, if Disney offsets the cost what they charge to members for Bell Services by the $6 fees then it is clean, and you'll see that show up on your dues statement likely or a lowering of the Bell Service fees charged. This is done for parking, fees Disney collects on the rooms it rents through breakage show up as income on it. Also if the $6 is pushed as a mandatory "gratuity" then because it goes directly to an employee it wouldn't be offset from your dues.