Since they're just used to get items that seem to ultimately be account bound anyhow, why not make the event tokens account bound, rather than character bound?
Add:
Also as the event going on at the time this suggestion was made shows, they are able to set event purchased items so they can only be purchased a certain number of times per day, per character, or per account. Such a system already allows them to regulate the volume each item could potentially be acquired by, which addresses the only concern people have really voiced against the idea.