It's because she's looking for the prince. Basically since she's close to the prince and if he insulted her, she could tell Zen to punish him if she wanted to since she knew Zen. It's a very good comeback because you know who's is who's status, and with the right person it'll shut them up real quick lest they want to be punished.
It's because he told her to lie on a bed and wait for the person she was looking for. He said it to insult her, but she said she was looking for Zen, which made it clear that by saying that Yuuha had also insulted and disrespected the prince too in the public, whom should've been respected by everyone. It's a clever comeback, actually :))
http://www.mangago.zone/read-manga/akagami_no_shirayukihime/mh/v10/c047/28/
So this scene here, it seems like Shirayuki apparently roasted the Yuuha guy, but I don't see how her response to his bitchy ass comment is a good comeback? Someone explain please?