Ah. I'm still a little sad that Shoutbox singalongs are impossible now, but this rule is understandable. Like Geo told me, it tends to completely take over the Shoutbox - the essential definition of spam around here. People who aren't participating might feel left out or annoyed.
And besides, it's really not too strict. Just a line or two is usually fine. I quote song phrases all the time, when a situation brings one to mind, and it's never been considered a problem in the Shoutbox.
It's just that getting carried away with it ends up turning spammy. P: