Default calendar for Todos shows all listed calendars grayed out
Calendars are type-specific. Each calendar may contain either Events or To Dos, but not both.
When creating a Todo or picking a default calendar for Todos, you must first ensure you have a Todo Calendar available. BusyCal does not support creating one for Google accounts, for instance, but does support creating ones for iCloud / Fruux.
When picking a default calendar for Todos, you may find most of your calendars greyed out, this is because these are all Event calendars. You need to pick a Todo Calendar from the list instead, and if one does not exist, you need to create one. In iOS you can only do this by creating a List in the "Reminders" app. On macOS you can create one using BusyCal's File Menu > New Calendar > Account > To do Calendar.
You can learn more here:
set calendar for to dos. Default todo calendar. Default calendar for Tasks. Pick calendar for To do's.