OR -- to use your example -- they already pad their budget by determining that cost per day per guest is $15 PLUS an additional $2 per guest to cover contingency or incidentals. So they charge guests $17.
49 guests come into the park at $17 each = $833
Actual cost to Disney for those 49 guests = $735
Extra available to Disney for "pixie dust" = $98
Disney builds Guest Recovery into the budget on the front end. The price you're paying today already includes the free ticket the OP got last week. To say that "they have to pay for it somewhere" is true. But that's true of everything at Disney, from adding another trash can to Main Street to adding another character experience in DAK. You can't say that Disney is raising prices because they're giving away free stuff. The giving away of the free stuff is already factored in.