Aha! I think I've figured it out...
So it looks like the bug in this case may actually be the ability to edit that filter. Since it's one internally-created by SmarterMail and linked to the "Block/Unblock" command, I can see why it would not be restricted from modification.
If you click on an email address in the message header and block the sender here, it gets added to the Internal Blocked Senders list. If you block a user this way though, the filter conditions and actions cannot be adjusted - only the email address list of blocked users can be changed. However, when you right-click on an email address and choose Block Sender here, you CAN modify the filter's actions (Caveat: you can only edit this filter from the right-click shortcut if you blocked your very first user this way. If a Internal Blocked Senders filter is already there from using the message header shortcut, it'll be added to that unchangeable filter.)
In short, it looks like the two links to block a user from an email (right click shortcut and header shortcut) are using two different functions when they should share the same functionality. If you delete all Internal Blocked Senders filters (copy the email addresses first) and block a user from the message header instead, you'll see how this option should be functioning.