well the legends are doubleshot, so its completely impossible for them to rub off
so i guess one of a few things could be happening
either, somehow you are transferring plastic residue from other parts of the cap on top of the legends, hence giving it the appearance that it is partially rubbing off, perhaps transferring oils or substances from your hand that is reacting with the purple plastic more so than the other plastic or a number of similar things with the same result
its very hard to comment without seeing them in person
id suggest a couple of things
firstly, is it happening across all the purple legends, or is it selective, or perhaps on the high traffic areas of typing
i would try taking the affected keys, and leaving them in a bowl overnight, starting with some warm water and a denture tab
hopefully that way we can see if it is something over the surface, or if it is something that has actually affected the plastic permanently, so we can at least eliminate certain things, and work from there
ive had my set on my daily since they came in, and have really heavily inspected it today and come up with nothing that looks anything like that
however obviously im in different weather and heat conditions, and most likely use different cleaning products etc