AM mode was the default mode for EasyVPN as its faster to establish, it
uses 3 messages instead of 6 messages to get the tunnel up. Now when to use
MM or AM is your design decision. The problem of MM messages isn't only
speed but computation overhead as well because you need to hash/encrypt
more messages than AG.
Because the initiator doesn't have Nr, KEr, CKY-R yet, hence, no SKEYIDs
generated to decrypt AM_2. If AM_2 is encrypted, initiator won't be able to
read the payload and get these values. Once it receives these values in
AM_2 as clear text it can generate SKEYIDs and verify the hash.
An attacker can generate fake AM_1 packets and get AM_2 packets with the
hash value and Nr, KEr, CKY-R. This way the attacker will have N, KE, CKY
for both sender and receiver along with hash that allows him to derive
SKEYIDs and later pre-shared key
