Most restaurants do have published nutrition information, just maybe not right out in plain view. There's a book called Calorie King that's pretty helpful - it lists restaurants and the nurtition info for their foods.
What you're suggesting isn't always practical, especially for small, non-chain family owned places. It would be really expensive for them to have all of their food evaluated, which drives up prices. And those smaller places tend to have healthier food than the chains, which would then be even more expensive. It's also can be a problem because preparation can vary - the amount of sweet onion sauce on my subway sandwich is always different depending on where I go and who makes the sandwich.
I think it's a better idea to look into one of the calorie/nutrition fact books, or ask your diabetes educator for one (mine's always pushing new versions at me), or you can look on restaurants' web sites. It's also important to learn to guage what a cup of something looks like, and memorize the carb info for basic things like rice, potatoes, a hamburger bun, etc, and use guessing skills. The more you try things out, experiment a little, and look at nutrition labels that are available, your guessing skills improve remarkably.
The basic idea that nutrition guides are important is good, but with all of the push for healthier lifestyles lately that information is out there. We don't really need to mandate that everybody should have to publish it.