Correct, what you are asking for is something that as is stated is not allowed. The thing is, its never just *you*.
You don't like the policy because it impacts you, i get that. But fair or unfair, its the policy your SIL agreed to follow. Do other conventions do something different, sure. Do some do something similar, sure. You bring up the conventions in your area don't have a policy like this, good for them. It still doesn't make the policy the WORST ever. All it does is make it the policy Gencon chooses to follow. I didn't pick similar to gencon conventions (well origins and maybe Dracon Con) but I also included other industries to point out that there may be other factors we are not aware of that make the conventions choose this type of policy. Unless they are all a part of some vast conspiracy (and I am making a joke there).
Of course this entire discussion could have been avoided had you simply had the badges/tickets shipped to you (as recommended by Gencon)