You could look at the 2% rebate as a cost of the membership revenues, but that isn't how the company, generally accepted accounting principles, or I look at it. There is a reason rebates are always recorded as reductions in revenues and not a cost.
Also, its probable the 2% incentivizes additional purchases by exec members, so logically speaking the gross margin on those additional purchases would be a full offset against the rebates even if you did decide to make your own accounting rules.
If you think something in the 90% contribution margins for memberships is more appropriate (to account for payment processing and the minimal staff time to sell memberships) great, whatever, doesn't really make a differencr. The business exists to collect membership fees, not earn margins on sales. Anyway, we're way off topic for a superstore thread here so I'm probably done, and you're welcome to have the last word on the matter.