When a customer visits “Home> My account>My vouchers”, the customer can see ALL cart rules ever created that match the customer group.
For example, if I want a voucher to be valid for all customer groups, but I want to distribute it only to people on Facebook, this feature/bug prevents that. Not only will the intended recipients on Facebook see the voucher when I post it there on my Facebook page, any customer who visits their account and clicks on “My vouchers” will see it, regardless of whether they use Facebook or not.
As noted by @SLiCK_303, this only happens where a customer group is selected for the voucher.
The discussion we had about this is here: https://forum.thirtybees.com/topic/1156/cart-rules-vouchers-seen-by-all-customers-is-this-a-feature-or-a-bug