The 'Comment viewable by' option displays all groups the user is a member, seems logical. But when the issue itself has been restricted by a security level the choices might not be logical.
For instance: a security level 'restricted' has been defined containing usergroups 'groupA', 'groupB' and 'groupC'
A user from groupA is also member of groups 'groupD' and 'groupE'.
When an issue has the level 'restricted' this user can add a comment and set the 'Viewable by' for 'All users', 'groupA', 'groupD' and 'groupE'.
It seems to me that 'All users' would translate to 'groupA' and the choices for 'groupD' and 'groupE' would not be usefull because members of those groups have no access to this particular issue.
So this option should not be available in this case.

Is the user also member of groupB it seems logical the 'Viewable by'
would display 'All users' (being groupA and groupB), groupA and groupB
So the comment can be restricted to a particular group within the security level.

Does this make any sense?