I, and most tarot users (readers and querents) don't see Tarot as a religion or strictly speaking, as a religious tool - at least, any more than it is a tool for business, romance, home buying or gambling. In other words, it's a tool which can be turned to many purposes, including religion. We might as well say that stem glasses should be banned because some use them as religious items (chalices). (Though I support the banning of knives from the workplace - it's not because I and some others use them as religious items).
Pagan X - since you are the boss, you can, of course, decide what constitutes a religious tool in your own workplace. But for my part, I would say your decision has more to do with conciliating your employees and keeping them happy that any strict dogmatic decision (and as such, it is to be commended).
Out of curiosity - in a quid-pro-quo, and in order follow a coherent policy, would you ban people wearing crosses, yarmulkes, scarves or turbans btw? I don't know the law in the US, but in many countries this would land you in hot water with the anti-discrimination enforcers.