Android phones used to only support one calendar via ActiveSync. Therefore, we introduced a white-list for devices that support multiple calendars and for all others we merge all calendar's events into one calendar.
Nowadays, most devices (even recent Android phones) do support multiple calendars and users frequently run into problems with only seeing one calendar.
Therefore, we should improve our support for multiple calendars. I see these options:
# Switch from a white-list to a black-list
# Make Folder-Merging configurable in ActiveSync settings
# Scrape Folder-Merging altogether