One of FeedBlitz’s (rarely used) features is to allow subscribers to be completely anonymous.
Firstly, they are extremely rare. Secondly, if we blocked the subscription the subscriber, if they wished, could use some hotmail, gmail or yahoo account that, although visible to you, would still not identify them (e.g. firstname.lastname@example.org is just as anonymous, right?). It’s simpler just to allow those who want to be stealthy to remain that way; after all, everything on your email is effectively public once it has been emailed out, since it can be forwarded anywhere.