The compulsory buying of an all inclusive plan when you visit your timeshare property is very rare. In most cases, as a timeshare owner you are likely to want to cook meals in your property, taking advantage of the kitchens and dining facilities that are common features of most timeshare properties. The idea of buying a timeshare is that you treat it like your second home, not an all inclusive hotel.
Where the confusion might lie is in the fact that many timeshare resorts also cater for hotel guests who do not own a timeshare on the site. Often, these guests can book an all inclusive package or a room only plan. Timeshare owners are then offered the chance to purchase an all inclusive plan to enjoy the resort fully. This is not usually obligatory and many resorts allow timeshare owners the option to buy vouchers by the day of three days at a time etc.
However, the key here is that all inclusive fees are not commonly compulsory for timeshare owners.