Yours is a very good question. The entire subject of kitniyot is riddled with controversy, and as well with uncertainty, as was recently the case with quinoa.
The custom of avoiding kitniyot relates to the making of bread from these ingredients, and the fear that allowing its consumption on Pesah would lead the unschooled to have actual bread.
For the same reason, seeds such as sesame, which could be ground into flour, became problematic, for fear they would be mixed up with other seed-like grains. Remember that even within the Ashkenazi community, much was dependent on the custom that was embraced. Some families never embraced the sesame stricture.
In the larger picture, Kitniyot expresses the concern of the "religiously aware" community for those not as religiously aware. That everyone who knows must keep away from something so as not to mislead those who do not know is a most eloquent expression of sensitivity for everyone in the community.
In the to-and-fro regarding kitniyot, we should not foget the big picture message that we are all united, and should all be concerned for each other.
I'm not sure how the topic of eating kitniyot (legumes) on Pesach fits into the general theme of Jewish values, but I'll answer your question anyway.
Kitniyot is the Hebrew term for the category of food known as legumes. They include rice, corn, soy beans, string beans, peas, lentils, mustard, poppy seeds... and yes even sesame seeds as you ask about. I don't think people are making flour out of peas, lentils, mustard or poppy seeds either by the way.
None of these products can become chametz, which is forbidden on Pesach, and yet most (a decreasing number I presume) Ashkenazi Jews do not eat them on Pesach because an Ashkenazi rabbinic authority ruled that they look like chametz products and could therefore be confused as such.
In other words, it's difficult to tell the difference between flour made from rice and wheat flour, which would be chametz. It seemed like a safe idea to just prohibit all kitniyot.
Today, in Israel it is much easier for those who eat kitniyot on Pesach because of the labeling on food products. In the Diaspora there are not as many products labeled 'Kosher for Passover for those who eat kitniyot'. More and more Ashkenazi Jews are beginning to eat products made from kitniyot on Pesach, including soy milk and hummus.
It is important to add that while there is a law barring Jews from even owning and benefiting from chametz products on Pesach that is not the case with kitniyot. Ashkenazi Jews who do not eat kitinyot on Pesach may still feed their pets products that contain kitniyot.
It seems that there are two Jewish principles considered here regarding this subject matter. The first is known as mar’eit ayin, which literally means the “appearance to the eye,” as in, the perception of customs and behavior by others. It is of tremendous importance in a world where “perception is reality,” as people say. The concern with sesame seeds, as for many other foods placed in the category of kitniyot, is that, even though the foods themselves may not be directly connected to foods that are explicitly forbidden, they may appear to someone, i.e. someone may see the food and think that it is a forbidden food.
The second principle here is the Talmudic concept of siyag l’Torah, literally meaning “a fence around the Torah.” This concept is, essentially, erring on the side of caution. If there is a case or circumstance—in this case a food item—that one is unsure of its official status, and/or that a passerby may be questioning or doubting as to its legality, then we build a “fence” around the law and place it squarely in the category of “forbidden,” as it is better to abstain from something questionable rather than partake of it. Thus, especially in the Ashkenazi traditions and customs, you will find that most interpretations fall on the side of a stricter reading, so as to prevent people from unintentionally breaking any halakhah.
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