There really is. The fact that you either don't realize this, or care to admit it, is cause for concern. The email being sent out to your users' contacts is highly personal, and one very few people would be comfortable with.
> If someone doesn't read the line "this will send a message" there isn't much we can do about that.
How far would you extend this logic? That is to say, how much can be justified by saying someone "missed the fine print"? In my opinion you clearly crossed a line here, just not a legal one.