I think its this false entitlement mentality that everything a company sells should be made as widely available for an expected cost. On one hand companies love these sort of behaviors because they're fostered by customers that will buy almost anything the company puts out. On the other hand, they can't come out and just say 'its our company, we can run it how we want, that's really how capitalism works" because it would alienate said costumers, although they'd probably get a lot less hate mail.
Con exclusives are a fun way to make the event special to those who make the trip and show up, I totally see why WOTC thinks its a good idea. Attendance selling out every year is directly tied to them making the event feel exciting, fun and unique. Its unfortunate that ebay and such exits, but its just a necessary evil in our world. Specifically eliminating the packet because you're afraid of resales on ebay is focusing on the wrong costumer. Most people will keep them in their collection as a collectible as well as a reminder of how much fun they had at gencon.
I think there's also a bit of a hive mind mentality that there MUST be something evil in every decision and product WOTC releases...