"Jewish dietary law in any case doesn't come under the ten commandments, and is a cultural aspect of judaism."
The ten commandments are only the ones which most people are most familiar with. If you try reading and understanding the remainder of the Torah, you will discover that there are five books, including Genesis and Exodus, which is where most people's knowledge seems to stop. By tradition, the content of these 5 books was handed from God to Moses and, thence, to the children of Israel. The last 3 books consist mainly of the laws which the Israelites are supposed to follow. There are several hundred of these laws. Each is important and they deal in issues of life and death, and community relations, as well as public health and sanitation.
Many of the laws do not seem to make sense to the community at large now. During times like the Middle Ages, they were often the cause of Jewish survival when those around them were dying of various communicable disorders, such as typhus and waterborne disorders. This difference helped create a community which those around them, mostly Christians, saw as practicing magic, instead of common sense. This identity has allowed the Jewish community to survive despite frequent attempts to eradicate Jews as a people and as a culture.
These rules are very slow to change and will not change unless a positive reason is shown for them to change. Two weeks ago, an orthodox Jewish congregation appointed a woman as its rabbi in all but name. It is a small but significant adaptation to the needs of the community.
In the case presented here, the sins have to be examined in terms of the Kaper Lanu prayer recited on Yom Kippur. These sins include sins committed in ignorance as well as sins committed intentionally. A sin is a sin, whether it is quitting smoking and regressing or unknowingly eating unclean food. The result is the same--a sin, a failure of a moral duty which a person has sworn, even if only to oneself. Having discovered the sin, it is up to the person to improve his or her ways and lead a better life.
The reason the community is fasting is as a duty to the larger community. There have been countless events in the past, such as plagues or invasions, [Sodom and Gomorrah and Noah's Flood come readily to mind] which are associated with moral failures within the community. These events were often what made the prophets famous in their efforts to clean up the community, often against public opinion that their ideas were not needed.
That the community is willing to atone for an unintentional sin is laudable. They are showing the universalism for which Judaism is famous. "Am I my brother's keeper?" They believe so and are acting on it.