And when you conect to msn with pidgin or any other free software you connect always thru microsoft servers, other thing is the protocol they use is free, in this case xmpp, so you can get access to their network, and microsoft doesn't like this much because they frecuently modify the protocol a little to **** up with people like us who don´t want tu use the original msn messenger program.

To see whattsapp protocol you can do with a MITM atack, and then analizing the capture in detail with wireshark, once you have a conversation captured in wireshark it will be in theory easy replicate the protocol.