Unfortunately, even in this day and age, that is true, albeit somewhat scary.
While I can see where the DoS stuff is coming from, what about this?
Channel is NOT +S :
Chanop sets +S - IRCd runs a quick check over who's in the chan, if they are NOT +S (SSL), then it removes them forcibly from the channel and prevents any non SSL joins, even through invite.
However, setting +S/-S plenty will cause the IRCd to have a small headfit as it checks each client over and over, only way to prevent that is to limit just how quickly you can perform a MODE change (such as +S), however then we get into a whole area of debate and general viper-nest stuff... May just be quicker (and easier) for us to leave it as is, at least as it is currently going +S/-S cannot DoS the server, which is more important IMO than the contents of a channel.
However, I just like to give options, even if they ARE fraught with pain and suffering... much like any other day at work really