As of 1 March 2012 all apps submitted to Apple's Mac App Store must implement sandboxing.
Apple told developers that they will have to implement sandboxing on their Mac App Store submissions as of that date.
As noted on TUAW, the sandboxing requirement is likely to ruffle a few feathers. However, sandboxing is a better security approach. It certainly isn't perfect, but the requirement isn't too far out of bounds.
Sandboxing isolates untested code changes such as malware and effectively quarantines it.
Apple said in an announcement:
The vast majority of Mac users have been free from malware and we're working on technologies to help keep it that way. As of 1 March 2012 all apps submitted to the Mac App Store must implement sandboxing. Sandboxing your app is a great way to protect systems and users by limiting the resources apps can access and making it more difficult for malicious software to compromise users' systems.
Via ZDNet US