I think you have located the correct place for the "issue". Not sure why it wouldn't be allowed though, as if tabulate didn't properly support multiple response columns it could just be treated as Nominal, for example how Graph Builder does it in JMP16 (in JMP17 Graph Builder will have some kind of support for multiple response New in JMP 17
Better option would be of course to have proper support for Multiple Response in Tabulate and I would think it will be added at some point (maybe create a wish list item for it?).
-Jarmo