The restaurant might be making an awkward effort to accomplish something that you might think is actually worthwhile. Generally, labor regulations provide that only service staff who customarily receive tips can share in tip pools. That means waiters, waitresses, and bussers, but not the chefs or dishwashers. http://www.dol.gov/whd/regs/compliance/whdfs15.pdf
In most restaurants, the waiters or waitresses keep all the tips, though sometimes they share with the bussers as part of a pool. This creates a massive income disparity where the tipped service staff in most busy restaurants make a pretty good living compared to the back of the house staff that doesn’t share in tips. This creates resentment between the front of the house workers and the back of the house. It doesn’t make for a well-run restaurant.
But who is doing most of the work in a buffet restaurant? Not the waiters and waitresses. I’m betting the bus staff keeps busy but the heavy lifting is being done by the chefs and dishwashers.
In a buffet restaurant, because the wait staff doesn’t need to spend a lot of time explaining the menu, taking orders, etc, the restaurant manager can and does conserve on waiter/waitress labor cost by spreading a a small wait staff across a lot of tables. So, even if the tips in buffet restaurants aren’t as good per table, the wait staff can still make a fortune giving indifferent service to a lot of tables at the same time.
So, if you want good staff to keep the buffet running well, you need to pay the back of the house better. And, since you expect a lot less of your waiters and waitresses, you could live with less capable waiters and waitresses. So, ideally, the restaurant manager would find a way to supplement the wages of the back of the house staff, even at the expense of the wait staff. That brings us to a mandatory service charge. With it, the restaurant manager can raise the wages of the chefs and dishwashers, while finding perfectly adequate, lower cost, wait staff that are maybe less professional than the servers in high-end restaurants. Since they no longer receive tips, the employer can’t take the tip credit and has to pay them at least the “full” minimum wage. The restaurant you went to claims they pay a living wage which probably means at least $10 per hour. The wait staff may not like losing the tips, but they like that their income is more predictable.
Why not just raise the prices 9% and dispense with the service charge? Because customers who don’t pay the service charge will feel compelled to tip anyway, recreating the tip pooling problem they were trying to avoid. It also means that their advertised prices are 9% higher than their competitors, and no restaurant manager wants to offer the same product at a higher price.