Adding a tip for large parties has been a common practice for decades—long before the current trend of adding tips for everything.
Whether you agree with the idea of tips or not, that’s the system we have, and in that system large parties very often take up far more resources at a restaurant than smaller ones. They take more time to leave, they clog up the kitchen (as all dishes need to come out at the same time), they often occupy multiple servers at the same time (when a normal table takes only one), and very often don’t tip at a level anywhere close to the standard 15% (with so many people, everyone thinks they can get away with a low tip and others at the table will make up for it). Servers get charged taxes as if they received those tips, so getting shorted actually costs them money (paying taxes on money they didn’t receive).
This practice is different than dark patterns, as it’s actually addressing a real problem with the customers that’s been known be restaurants for a long time. It’s not the same thing as trying to trick or guilt people into tipping.