It's been discussed before. A lot of tourist destinations have this policy in effect and have had the policy in effect for holidays. It's no different from hotels charging extra for weekend and holiday nights.
If it bugs you that much, don't eat at buffets or simply change to eating at a buffet at lunch. That's what we did at the Biergarten. Same food as dinner and the lunch price with the surcharge was the same as the dinner price without the surcharge.