They might try a DMCA case under the circumvention clauses, no? "We have said your addon may not run and we have used access controls to prevent it from running, which you have circumvented." It's somewhat similar to Lexmark v. Static Controls (which they lost), but isn't quite the same since Lexmark wanted to prevent any third party from providing cartriges, whereas Blizzard would be preventing select addons.
Edit: However, I believe that Blizzard really should have an SDK/API licensing agreement with addon authors.
Edit again: If it isn't actually enforceable, then it's really not in Blizzard's best interests to keep it as is. If they to try to go to court and get thrown out, they really do have a problem then.