It may be one of the things listed under Proposition 65 in California (a regulation which says anything with even the slightest chance of causing "cancer, birth defects, or other adverse reproductive outcomes" has to be reported on the label) or a similar law. This rule means that even things with "no significant risk" need to have labels, for example Maraschino cherries "cause cancer". If you eat million of these cherries, you may get cancer from it, however, with any number of cherries you could conceivably eat, you will be fine.
I don't know, but it may be something covered by this law. However, it would not be used if there was any significant risk of cancer from it.