Many of the laws or rules are based on common sense health codes and laws we have today. A lot of politics come into play also, Example my on premise rabbi from the (OU)
union of orthodox rabbis will not permit meat into the building unless a letter accompanies the meat, from the rabbi where the meat came from(wholesaler). Therefore assuring the fact it is kosher, but also guaranteeing the other rabbi a job.
You usually can't by from a place that does not adhere to the sabbath. You can't mix dairy and meat at same time or same meal. Seperate sets of dishes for meat and dairy even in some cases seperate dishwashers. Another set of dishes for passover. You can circumvent this by using glass plates, which are non pouris. Seperate pots and pans for cooking and in some cases seperate kitchens. Thats why Kosher caterers charge more.:beer: