I'm not sure why these Flash interfaces exist. More than likely someone who's good at running a restaurant isn't going to be know a lot of technology and will get roped into the most expensive flash-based design they can afford. Items like usability, mobile friendliness, etc never come up because the owner doesn't know or care about it and the designers are too invested in Flash to try anything else.
Is this hurting their business? I'm not sure. I never go to restaurant sites. I visit yelp instead. I only do takeout from places that have good systems in place. Its a competitive advantage now to take online reserverations and have a menu I can view on my phone. Considering something like 80% of all new restaurants go out of business after their first year, it might make sense for these owners to stop being so pigheaded about technology and ask themselves if that slow non-mobile $20,000 Flash site is really serving their customers.
This is the same battle we had with restaurants being cash only 15 or so years ago. The ones that embraced credit cards seemed to have flourished and the cash-only place is either gone or has degenerated into a dive. I think the lower tech your industry is, the most room there is for disruption. These blue-collar places really need to consider the big picture here.