Having given the matter some thought, it has suddenly occurred to me that it is not actually a copyright question. It is a contractual question. It is the contract (the EULA) that stipulates this -- and is therefore enforeceable under those terms (less wiggy than IP rules). And, as noted, it really is a matter of their choosing to enforce it accordingly. So long as your items do not directly compete with their items, it should be fine.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)