Why is ALTER authority needed when we use the Windows Client to delete a JOIN/SUMMARY Object?
Deleting a member only requires UPDATE authority when deleting the member through TSO panels.
ALTER is necessary for DELETE data set. The UPDATE is sufficient, when a member of the data set should be deleted.
However, if a user with UPDATE authority deletes all the members, the effect is more or less the same as if the whole data set is deleted from a user having ALTER authority.
From the security point of view requesting ALTER authority not only simplifies the process, the ALTER is necessary for some actions with the data set, like Archive, Migrate, Move, or Delete. By default this is recommended.