Why do full previews expire?
When you visit a shared bin URL you might see this message across the top:
In particular the message:
This bin was created anonymously and its free preview time has expired
If a bin is created by an anonymous user, the full output (that is: with the JS Bin editor stripped away) is only available for 90 minutes after creation. After which the URL will redirect to the editor view.
If you're (freely) registered anyone can always view your full output. This change took effect on all anonymously created bins as of July 3, 2014.
JS Bin has long been a target for spam and phishing and this is first change that would outwardly impact our users. After speaking to many of you, and taking your feedback, this security change was the right balance between addressing the issue of spam, but also maintaining flexibility for those of you who use JS Bin in a teaching environment.