1) you can never assume a recipient ever has their phone on, with them and in a non-silent mode 2) users can disable push notifications for individual apps anyway
That said, the reasons you mention are pretty edge cases. Instead of just hating on it, just stop and think about it for a moment without prejudice. When you email someone you don't expect them to read it immediately, but when you send a text you at least expect them to know they received something, no? (If you say no to this, I really have nothing else to say anymore, you would be just denying the reality) That's what Facebook wants to be. They're competing with SMS, not email.
Shenanigans!
Also, if they're supporting desktop clients I again call shenanigans on your argument.