Apps that use Dropbox integration to allow their users to share personal files between devices, are getting banned by Apple. For the integration to work, the Dropbox App must be installed. If it's not installed, the apps will redirect the user -- through Safari -- to the Dropbox page so that it can be downloaded. From here, there is a chance that someone could purchase something, therefore Apple has banned these apps on the remote possibility they could be missing out on that 30% cut.
From the article on PcPro:
The decision has been met with anger - and predictable sarcasm - from developers posting on the Dropbox forums. "Apple should reject all web browser apps because they can take you to a page that lets you purchase stuff," writes one contributor. "Go Apple! Crack down on all commerce!"
Dropbox is working with Apple to resolve the situation, according to a report on CNet.