Stamps use CounterParty under the hood, which simply switches to bare multisig once the payload goes over 80 bytes. With the proposed Bitcoin Core change to policy, or just considering relay reality, CounterParty could stop doing that. But afaik nobody is maintaining it, and the inefficiency is a feature for some of its users (stamps).
Discussion
You're right, those are in another class altogether. Upvoted your post and answer fwiw :)
