That is correct. If you have ALTER on a schema, that implies ALTER on all objects on that schema. This can be counter-acted with DENY but that is of course hopeless.

That's why you can't have these user-defined tables in dbo, but as I suggested in a separate schema. The users will have permission to alter anything in that schema, but they will not have permissions to create anything but tables, so there will not be anything else do alter than then tables.