On Android, declaring an intent filter for an activity in the AndroidManifest.xml file means exporting the activity to other apps. If the activity is intended solely for the internal use of the app and an intent filter is declared, any other apps including malware can activate the activity for unintended use.
In the case of twicca app, by launching twicca's activity, another app that does not have permission to access the SD card or network could upload images or movies stored on the SD card to a SNS service with twicca user's twitter account.