I just bought a box of 100 sandwich bags and on the box there is an (U) symbol. This is neither unique nor uncommon. Why do marks of hechsher appear on non-food products? Is there anyone in the world looking for such a hechsher or making plasticware decisions based on it?
"Quite frankly, we were astonished to learn of the rather extensive use of stearates and other tallow based chemicals in the production of plastic food-contact materials. These chemicals may be added to plastics in various formulations at concentrations as high as two percent with the intent and knowledge that they will migrate to the surface, thereby interacting with the foods they contact. Due to this migration and interaction the FDA requires that all additives used in plastic food-contact materials be of an acceptable food grade quality."
In addition, Rabbi Zushe Yosef Blech reports that some non-food products that come into close contact with food, such as foil and parchment paper, may contain release agents made from animal fats, or food-grade but non-kosher lubricants used on the machinery may have come into contact with the product and come to be exposed to food when you use it. Therefore, kosher supervision is required for these products, too.
(I would assume it's so one can be assured no non-kosher residue is on the surface of the bag. But I'm posting this answer mostly for the following.) As to "Is there anyone in the world looking for such a hechsher or making plasticware decisions based on it", well, doubtless. Even if all rabanim who know what they're talking about will tell you it's not necessary (and I don't know that that's the case here), surely someone will look for it.
From the N.Y. Times:
Only about 15 percent of people who buy kosher do it for religious reasons, according to Mintel, a research group that last year produced a report on the kosher food explosion. The top reasons cited for buying kosher? Quality, followed by general healthfulness.