Actually they are, they just display the name as "*" and no topic, so all you can see is the number of users. Even the RFC says so:
The list message is used to list channels and their topics. If the <channel> parameter is used, only the status of that channel is displayed. Private channels are listed (without their topics) as channel "Prv" unless the client generating the query is actually on that channel. Likewise, secret channels are not listed at all unless the client is a member of the channel in question.
("Prv" is actually how it was displayed in the irc2.8 client, the server still sent it as "*")