Why WhatsApp allows anyone in the world to know whether I'm online
This haunts me and many websites selling services to monitor users.
* I'm not talking about "last seen" feature.
As far as I know, WhatsApp builds upon XMPP, a protocol which also includes presence (the first P of XMPP). Good XMPP clients let you choose which contacts can receive presence stanzas. WhatsApp is not one of them (it also does not federate). Use Conversations instead.