How is that unexpected or unwanted compared to the original argument? You have a right to say whatever you feel like, at whatever volume you feel necessary. The land owner has a right to kick you out of her business for whatever reason she deems appropriate; this includes private conversations.
I've been to places that won't seat individuals, only groups. I've been to places that won't seat you if you wear jeans.
I think most people would be pretty upset if they were ejected from a restaurant half-way through a meal for the content of an otherwise-intimate conversation, in contrast to the case of having a quiet meal disturbed by an unwanted intruder. There's an implicit contract in a restaurant that if you're seated and conduct yourself with decorum that your interaction with the restaurant operators will center on the exchange of food for money.
I've been to places that won't seat individuals, only groups. I've been to places that won't seat you if you wear jeans.
The key difference being that you haven't settled in to enjoy your meal in either of those examples, although you may be disappointed to be turned away.
It has nothing to do with being upset. It has to do with the business owner's right to kick you out for pretty-well any reason s/he sees fit. The person who is loudly interrupting your meal will probably be just as upset for being booted for expressing his opinion.
Not being seated is the same as not being allowed to enjoy your meal, regardless of where you fall on the 'I've already been seated' continuum.
I've been to places that won't seat individuals, only groups. I've been to places that won't seat you if you wear jeans.