It doesn't matter if they send a courtesy message. This was done during specific times of crisis (or while they believed there may be more attacks forthcoming after an event). If the terrorists either don't notice or don't care about the warning, the security services can continue to listen to the terrorists co-ordinating their attacks; if the terrorists do notice, they have to make a choice -- stop co-ordinating with each other (potentially ruining their plans) or continue talking and thus inform the security forces of every stage of their attack.

It's also quite encouraging that this has only been used twice in 9 months; it shows they are using it responsibly and only in extreme situations.