I'm 99.99% sure it's a Cambridge / Boston ABCC thing ... An earlier draft of the contract had a full corkage waiver. (For various reasons (Boskone-From-Hell, etc.) Arisia probably would still have had an open / closed party policy). The hotel explained to us why they couldn't sign that and cited the Cambridge liquor licensing policy. Given I've had other hotels cite this in Boston / Cambridge and have had no problems getting full corkage waivers in other towns, I tend to believe them.

(The hotel is licensed a whole to serve alcohol. They are the only people who can serve alcohol by law in the hotel. Kind of like you can't bring booze into a restaurant with a liquor license.)