As for the taxes, I think he meant that if you buy something, the price on it is shown without taxes, therefore it´s misleading.
sales tax is levied by each local government, so the amount you pay is different wherever you go - some places don't have any at all, some places are very high. some states don't tax food or clothing. however, most products are produced and distributed nationwide, and marked at the same price wherever they are sold - usually set by the manufacturer (called the manufacturer's suggested retail price or MSRP). taxes are charged separately so that the government's taxation does not interfere with private competition for the pricing of goods and services.
it is annoying though
I have customers from Minnesota who are horrified that Wisconsin charges sales tax on clothing, while my customers from Chicago are delighted to pay half the amount of sales tax that they have to pay back home.
But in the USA the fast food seems to be the main source of food for everyone.
not true at all. most communities are divided into various zones (the local governments even legislate this for various reasons). there are residential areas, commercial, industrial, business, etc.
As a tourist - you probably spent most of your time in commercial or business areas - people work in these areas, they don't live there, so there are lots of restaurants for them to eat lunch, and for tourists to frequent.
Meanwhile, the supermarkets are generally located in residential areas where people actually live, so that they can buy their groceries near home.
Few tourists hang out in the residential areas, since there is nothing to see there but normal people doing the same normal things that normal people do back in whatever country the tourists happen to come from
again, it is annoying when you are travelling and can't find a place to pick up some groceries - but I assure you that most of us here in the US don't eat all of our meals at fast food restaurants - we buy groceries and cook at home just like people in every other part of the world